United States 

Department of 

Agriculture 

Animal and Plant 

Health Inspection 

Service 

Plant Protection and 

Quarantine 

February 11, 2019 

Version 2.3 

Guidelines for the USDA-APHISPPQ Weed Risk Assessment 

Process

Agency Contact: 

Plant Epidemiology and Risk Analysis Laboratory 

Center for Plant Health Science and Technology 

Plant Protection and Quarantine 

Animal and Plant Health Inspection Service 

United States Department of Agriculture 

1730 Varsity Drive, Suite 300 

Raleigh, NC 27606 
TP E-300, Ver. 2.3 February 11, 2019 1

Table of Contents 

Table of Contents .......................................................................................................................... 1

List of Tables ................................................................................................................................. 3

List of Figures ................................................................................................................................ 4

Section 1: Introduction to the PPQ Weed Risk Assessment ..................................................... 5

Risk analysis framework ............................................................................................................. 5

Usage and meaning of the term “invasive” ................................................................................. 7

Section 2: Overview and Interpretation of the PPQ Weed Risk Assessment (WRA) ............ 8

The predictive model ................................................................................................................... 9

Secondary screening .................................................................................................................. 10

Geographic potential ................................................................................................................. 10

Entry potential ........................................................................................................................... 11

Uncertainty ................................................................................................................................ 12

Summary of uncertainty ........................................................................................................ 12

Uncertainty Analysis ............................................................................................................. 12

The final product ....................................................................................................................... 14

Section 3: Conducting the PPQ WRA ....................................................................................... 15

Process 1: Gathering background information .......................................................................... 15

1-1. Verifying taxon identity and listing synonyms and common names ............................. 16

1-2. Familiarize yourself with the plant taxon ...................................................................... 17

1-3. Documenting the initiating event .................................................................................. 17

1-4. Briefly review the foreign and U.S. distribution of the taxon ....................................... 17

Process 2: Gathering the evidence ............................................................................................ 18

2-1. Familiarizing yourself with the questions and the Excel workbook ............................. 18

2-2. General approach to evidence gathering ....................................................................... 20

2-3. Reviewing the literature................................................................................................. 21

2-4. Gathering evidence on U.S. distribution and status of the plant taxon ......................... 23

Process 3: Answering the questions and describing Uncertainty .............................................. 26

3-1. Presenting and evaluating the evidence ......................................................................... 26

3-2. Answering the questions ................................................................................................ 29

3-3. Determining uncertainty ratings for questions .............................................................. 32

3.4. Entering the level of uncertainty .................................................................................... 34

3.5. The index of uncertainty ................................................................................................ 35

3-6. Tracking the process in the spreadsheet ........................................................................ 36

Process 4: Assessing geographic potential ................................................................................ 36

4-1. Gathering and preparing the data for input into ArcGIS ............................................... 37

4-2. Adding georeferenced and reported occurrence data to ArcMap .................................. 38

4-3. Assessing the global distribution of a taxon .................................................................. 38

4-4. Conducting Proto3 analysis ........................................................................................... 39

Final notes ............................................................................................................................. 39

Process 5: Assessing entry potential ......................................................................................... 40

Process 6: Uncertainty analysis ................................................................................................. 41

Process 7: Finalizing and assembling the WRA ....................................................................... 44

7-1. Reviewing the final outcome of the assessment in the Excel workbook ....................... 44
TP E-300, Ver. 2.3 February 11, 2019 2

7-2. Preparing and finalizing the WRA Word document ..................................................... 45

7-3. Peer review of the final WRA product .......................................................................... 50

Literature Cited .......................................................................................................................... 50

Appendices ................................................................................................................................... 54

List of Appendices .................................................................................................................... 54

Appendix A. Questions and scoring used in the PPQ weed risk assessment ............................ 55

Appendix B. Logistic regression model formulas ..................................................................... 59

Appendix C. Logistic regression model and decision thresholds for Low Risk, Evaluate 

Further, and High Risk .............................................................................................................. 60

Appendix D. Secondary screening tool of the PPQ WRA ........................................................ 61

Appendix E. Question-specific guidance and examples for the risk element 

Establishment/Spread Potential ................................................................................................. 63

Appendix F. Question-specific guidance and examples for the risk element Impact Potential 88

Appendix G. Question-specific guidance and examples for the risk element Geographic 

Potential ................................................................................................................................... 106

General guidance for recording evidence ........................................................................... 106

General guidance for answering the questions ................................................................... 107

General guidance for addressing uncertainty ...................................................................... 107

Specific guidance for each climatic variable ...................................................................... 107

References for Geographic Potential .................................................................................. 110

Appendix H. Question-specific guidance and examples for the risk element Entry Potential 111

Appendix I. Conceptual process for the uncertainty analysis in the PPQ WRA .................... 119

Appendix J. Work instructions for performing an uncertainty analysis (a stochastic simulation) 

using @Risk. ........................................................................................................................... 121

Acknowledgements ................................................................................................................... 123

Version Change Log ................................................................................................................. 124

 
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List of Tables 

Table 1. Description of worksheets within the PPQ WRA Excel workbook. 

Table 2. Comparison of the terms “invasive plant” and “weed” as they relate to the PPQ WRA. 

Table 3. Multiple-choice questions in the PPQ WRA. 

Table 4. Uncertainty ratings used to describe question-level uncertainty in the PPQ WRA. 

Table 5. Thresholds for different qualitative levels of uncertainty for the Establishment/Spread 

and Impact risk elements. 
TP E-300, Ver. 2.3 February 11, 2019 4

List of Figures 

Figure 1. Sample excerpt from the workbook of an in-progress WRA. 

Figure 2. Risk space of the PPQ WRA model as determined by the risk scores for 

Establishment/Spread and Impact Potential. 

Figure 3. Map of the United States, including Alaska, Puerto Rico, and Hawaii, showing the 

areas estimated as suitable for establishment for the plant taxon under assessment. 

Figure 4. Sample results of an uncertainty analysis for a plant taxon. 

Figure 5. GBIF map of the plant records associated with point occurrences for Heliotropium 

europaeum.

Figure 6. Framework for considering the extent of a taxon’s cultivation in the United States. 

Figure 7. Example of the “Risk Score Summary” section (cells A126-E134) of the “Screening 

Tool” worksheet. 

Figure 8. The general impact of factors on the level of uncertainty associated with an answer. 

Figure 9. Overview of the PPQ Proto3 model for assessing the geographic potential of plant 

taxa. 

Figure 10. Model simulation results (N=5,000) for uncertainty around the risk score for Hakea 

europaeum.

Figure 11. Representative uncertainty analysis charts for different levels of uncertainty in 

Establishment/Spread Potential (x-axis) and Impact Potential (y-axis). 

Figure I-1. Conceptual process of the stochastic simulation that is conducted as part of the PPQ 

WRA uncertainty process. 

 
TP E-300, Ver. 2.3 February 11, 2019 5

Section 1: Introduction to the PPQ Weed Risk Assessment 

The United States Department of Agriculture (USDA), Animal and Plant Health Inspection 

Service (APHIS), Plant Protection and Quarantine (PPQ) conducts weed risk assessments 

(WRAs) to evaluate the risk potential of a plant taxon becoming weedy or invasive and to assess 

where it might establish in the United States. Article IV.2 of the International Plant Protection 

Convention (IPPC, 1997) indicates that national plant protection organizations are responsible 

for conducting pest risk analyses to protect the plant resources of their country. USDA-APHISPPQ is the national plant protection organization for the United States and is responsible for 

safeguarding U.S. plant resources from pests including noxious weeds. The national regulatory 

authority to conduct WRAs is provided in Title IV – Plant Protection Act of 2000 (7 U.S.C. § 

7701-7786). 

In this document, we provide guidance for conducting WRAs using the PPQ WRA process. The 

document should be useful to risk analysts, as well as to customers, stakeholders, and trading 

partners interested in understanding our methodology and products. PPQ WRAs are initiated to 

address a variety of issues and situations; however, one of the primary reasons we conduct a PPQ 

WRA is to determine whether or not a plant taxon is a candidate for listing as a Federal Noxious 

Weed. A Federal Noxious Weed is defined as “any plant or plant product that can directly or 

indirectly injure or cause damage to crops (including nursery stock or plant products), livestock, 

poultry, or other interests of agriculture, irrigation, navigation, the natural resources of the 

United States, the public health, or the environment” (7 U.S.C. § 7701-7786). Federal Noxious 

Weeds are taxa prohibited or restricted from entering into or moving interstate within the United 

States. For transparency, Federal Noxious Weeds are listed as part of the Federal Noxious Weed 

regulations (7 CFR § 360). Except for plant species unlikely to contaminate import or export 

pathways, most Federal Noxious Weeds are also listed as Noxious Weed Seeds, which are weed 

species that are regulated in agricultural and vegetable seed entering or moving through the 

United States in commerce (see 7 CFR § 361). Any person or organization can petition PPQ to 

list a plant taxon as a Federal Noxious Weed, which may initiate a WRA. PPQ prioritizes taxa 

for evaluation based on the current demand for WRAs. 

Risk analysis framework 

The PPQ WRA process is consistent with international standards for phytosanitary measures 

(ISPMs) (ISPM Nos. 2 and 11), as well as regional (North American) standards for phytosanitary 

measures (RSPMs) (RSPM No. 32) for pest risk analysis (IPPC, 2011, 2013b; NAPPO, 2008). 

These standards identify three stages of pest risk analysis: Stage 1, initiation; Stage 2, risk 

assessment; and Stage 3, risk management (IPPC, 2011). PPQ WRAs address Stages 1 

(initiation) and 2 (risk assessment). Stage 3 (risk management) is beyond the scope of a PPQ 

WRA. 

PPQ WRAs can be initiated for different reasons, such as: 

 determining whether a plant taxon is a candidate for listing as a Federal Noxious Weed 

 evaluating plant taxa: 
TP E-300, Ver. 2.3 February 11, 2019 6

o that have been designated by PPQ as “Not Authorized Pending Pest Risk 

Analysis” (i.e., NAPPRA) 

o that are newly detected or that have expanded their geographic range in the 

United States 

o that are exhibiting invasive traits elsewhere in the world 

o when there is stakeholder concern of their risk potential 

o that have been requested for import as either a commodity for consumption or a 

plant for planting 

One of the steps in pest risk analysis is categorization, during which taxa are evaluated to 

determine whether they meet the defining criteria to be considered quarantine or regulated nonquarantine pests (IPPC, 2011). The purpose of categorization is to identify (i.e., screen out) 

species that do not meet these definitions before subjecting them to a lengthier risk analysis. 

However, because plant taxa for which there is no evidence of spread or impact elsewhere in the 

world can later become pest plants (IPPC, 2011; Whitney and Gabler, 2008), PPQ evaluates the 

risk potential for all plant taxa based on their inherent biological traits and behavior outside of 

their native range (e.g., Mack, 1996; Reichard, 2001). Essentially, in the PPQ WRA, 

categorization and risk assessment are combined into one process and used as a screening tool to 

categorize the status and potential risk of the plant taxon. 

The PPQ WRA predictive model (Koop et al., 2012) is a major component of the overall PPQ 

WRA process and evaluates the risk potential of a plant taxon. The term “WRA model” or 

“model” is used to refer to the predictive logistic regression model that quantifies a plant taxon’s 

ability to escape, establish, spread, and cause harm. The terms “PPQ WRA” or “WRA process” 

refer to the overall process for evaluating a plant taxon’s risk profile, including the portion 

evaluated by the predictive model. 

The completed PPQ WRA does not include policy or risk management recommendations. It 

categorizes risk and relates the risk scores obtained during the assessment to the dataset of plant 

taxa used to develop and validate the predictive logistic regression model (Koop et al., 2012). 

Based on the plant taxon’s risk scores, the PPQ WRA categorizes the risk as either “low risk,” 

“evaluate further” (moderate risk), or “high risk.” Because most policy decisions for cultivated 

plants are to either allow or deny entry into the United States, we recommend that taxa with a 

moderate risk potential (i.e., those with a risk categorization of “evaluate further”) be further 

assessed by some other process to help inform policy and decision makers. 

While the conclusions from the PPQ WRA are not official policy recommendations, the 

analytical and statistical methodologies behind these conclusions support management decisions 

of allowing entry to low risk taxa, denying entry to high risk taxa, and further evaluating other 

taxa as appropriate. The three possible outcomes of the PPQ WRA are similar to those of other 

weed risk assessment models (e.g., Pheloung et al., 1999; Reichard and Hamilton, 1997). Risk 

managers in APHIS-PPQ use results from PPQ WRAs in conjunction with other information to 

decide what Federal action may be appropriate. 

In general, if regulatory action is prudent for a pest, risk managers determine which mitigation 

options might reduce risk to an acceptable level. This risk management process corresponds to 
TP E-300, Ver. 2.3 February 11, 2019 7

Stage 3 of pest risk analysis (IPPC, 2011). For plant taxa not yet present in the United States, 

most management decisions will be to either allow or deny them entry. Agency policy and 

management decisions are summarized and communicated separately in a risk management 

document. 

Usage and meaning of the term “invasive” 

Terminology in the weed/invasive plant literature is inconsistent and imprecise, as words such as 

“weed” and “invasive” have subjective and sometimes ambiguous meanings (Richardson et al., 

2000). As with other studies that have developed and/or tested WRA models (e.g., Gordon et al., 

2008), Koop et al. (2012) relied on information available in the literature to identify U.S. plant 

taxa belonging to three categories of invasiveness: non-invaders, minor-invaders, and majorinvaders. In this usage, “invader” broadly refers to a taxon’s overall ability to establish, spread, 

and cause negative impacts (i.e., harm) and includes several components of risk recognized and 

used by the International Plant Protection Convention in its international standards pertaining to 

pest risk analysis (IPPC, 2011; IPPC, 2013b). 

In the PPQ WRA, the Establishment/Spread and the Impact Potential of a plant taxon are 

evaluated as two separate risk elements. The risk scores for these two elements are used by the 

predictive model to characterize the overall invasive potential of the species, in the broadest 

sense. When evaluating establishment/spread, a stricter definition of the term “invasive” is 

adopted (referring to a taxon’s capacity to escape, establish, reproduce, and spread throughout a 

landscape (Richardson et al., 2000). However, at the end of the PPQ WRA process, when the 

scores for both risk elements are considered, a broader definition of the term “invader” is used, 

because risk scores are related back to the plant dataset used to develop and validate the WRA 

model (Koop et al., 2012). 

If introduced into the United States, we predict that low-risk plant taxa are likely to remain noninvaders or non-weeds, while high-risk taxa are likely to become major-invaders or major-weeds 

after some indeterminate period1

. Taxa classified as Evaluate Further typically have intermediate 

risk scores between those of low- and high-risk plants. They pose a moderate risk and are likely 

to become minor-invaders or minor-weeds—that is, species likely to escape and naturalize but 

not expected to become widely naturalized or cause widespread harm. 

 

1

 The period of time after a species is introduced but before it behaves invasively is generally referred to as the lag 

phase of invasion (Kowarik, 1995). 
TP E-300, Ver. 2.3 February 11, 2019 8

Section 2: Overview and Interpretation of the PPQ Weed Risk 

Assessment (WRA) 

A PPQ WRA has three major analytical components: 

 assessment of the plant taxon’s Establishment/Spread and Impact Potential (i.e., its 

invasive potential) 

 evaluation of the sensitivity of the Establishment/Spread and impact risk scores to 

uncertainty 

 determination of the areas in the United States suitable for establishment by the taxon 

The PPQ WRA is conducted in a Microsoft Excel workbook (see Fig. 1), while the final report is 

written in Microsoft Word. In the report we describe the plant taxon’s risk profile and discuss the 

information considered in the evaluation. The report includes the following information and 

sections: 1) title page, 2) background and species information, 3) written and graphic summaries 

of the analysis, 4) discussion, 5) literature cited, and 6) a copy of the answers and justifications 

from the Excel workbook, including all evidence considered. 

During the development of a WRA, we gather scientific evidence and other information for 

answering a series of questions that characterize the risk posed by the plant taxon. Most 

questions require an answer of yes (= y), no (= n), or unknown (= ?). A handful of questions 

require multiple-choice answers of a, b, c, etc. The questions are organized into the following 

four risk elements: 

 Establishment/Spread (ES) Potential – 23 questions 

 Impact (Imp) Potential – 18 questions 

 Geographic (Geo) Potential – 3 variables with 36 questions total 

 Entry (Ent) Potential – 12 questions 

See Appendix A for a complete list of the questions for each risk element. Questions that are 

more strongly associated (i.e., predictive) with invasiveness and weed potential are more heavily 

weighted in the PPQ WRA (Koop et al., 2012). As part of the process, risk analysts are required 

to qualitatively categorize the uncertainty associated with each answer. Uncertainties are 

categorized as negligible (= negl), low, moderate (= mod), high, or maximum (= max) based on 

the quantity and quality of the evidence supporting each answer. 

For the ES and Imp risk elements, the Excel workbook automatically assigns a pre-determined 

score for each given answer, and then sums the scores to produce an overall score for that risk 

element. Risk elements have the following score ranges: ES = -25 to 32, Imp = 1.0 to 5.1, Ent = 

0 to 1. We do not assign scores to the Geographic Potential risk element. Instead, we use the 

answers to the Geo questions to answer three questions under ES and develop a map of the 

potential distribution of the species in the United States (see below and Process 4). 
TP E-300, Ver. 2.3 February 11, 2019 9

Figure 1. Sample excerpt from the workbook of an in-progress WRA. Evidence is organized 

under the “Notes (and references)” column for each of the questions. Once enough evidence has 

accumulated for any given question, the risk analyst enters an answer and uncertainty level 

(“Answer” and “Uncertainty” columns, respectively). 

The predictive model 

At the core of the PPQ WRA is a logistic regression risk model that describes the risk potential 

of the plant taxon being assessed. The model uses the risk scores from the Establishment/Spread 

and Impact risk elements to determine the likelihood that a given plant taxon will be a non-, 

minor-, or major-invader (Appendix B). These likelihoods are probabilities that sum to one (or 

100 percent, depending on how they are expressed) for any given plant taxon and are 

automatically calculated by the workbook. 

During the development of the PPQ WRA model, decision or risk thresholds were selected that 

maximized the model’s ability to correctly identify non-, minor-, and major-invaders while 

minimizing predictive errors. These thresholds can be expressed in terms of either the 

probabilities of invasiveness themselves (Appendix C) or the original risk scores for ES and Imp 

(Fig. 2). The decision thresholds create three risk rating regions that summarize the outcome of 

the predictive model: Low Risk, Evaluate Further (moderate risk), or High Risk. 

In the PPQ WRA product (i.e., the Word document), the ES and Imp risk scores for the plant 

taxon under assessment are superimposed on a graph displaying the risk scores obtained for the 

204 plant species used to develop and validate the PPQ WRA model (Koop et al., 2012). The 

decision thresholds that separate the Low Risk, Evaluate Further, and High Risk regions are also 

shown on the graph for reference (diagonal dashed lines in Fig. 2). This graph is one of the 

principal analytical outputs of the WRA. 
TP E-300, Ver. 2.3 February 11, 2019 10

Figure 2. Risk space of the PPQ WRA model as determined by the risk scores for 

Establishment/Spread and Impact Potential. Diagonal dashed lines represent the risk decision 

thresholds between Low Risk (left), Evaluate Further (moderate risk) (middle), and High Risk 

(right). 

Secondary screening 

Plant taxa that receive a risk rating of Evaluate Further (i.e., those with moderate risk potential) 

are subjected to a secondary screening that helps to reclassify some of the taxa as either High or 

Low risk. This process helps risk managers as they evaluate possible management options for 

these moderate-risk taxa (i.e., should the taxa be prohibited or should they be allowed entry into 

the United States?). During secondary screening, plant traits that are strongly associated with 

plant invasiveness are examined. 

While many different approaches can be taken with taxa that present a moderate risk (e.g., Mack, 

1996), the design of the secondary screening tool in the PPQ WRA is similar to the one 

developed for the Hawaiian WRA process (Daehler et al., 2004). The tool uses a short decision 

tree containing six questions associated with plant invasiveness that are based on life-history 

traits and behavior elsewhere in the world. The list of questions and a graph of the secondary 

screening decision tree are found in Appendix D. In the PPQ WRA, the results of secondary 

screening are reported immediately after the results of the primary risk model. It should be noted 

that even after secondary screening, some plant taxa may still remain in the Evaluate Further 

category. 

Geographic potential 

Unlike most other WRA models, the PPQ WRA does not consider climatic suitability within the 

Establishment/Spread or Impact risk elements (i.e., within the predictive model). The United 

States (including its territories) is climatically diverse because of its large size and the 

distribution of its land area across different latitudes. As such, we designed the WRA process to 

be climatically neutral so the model would not be biased against smaller climatic regions (e.g., 

tropical forests, Mediterranean) of the United States. Thus, the risk ratings reported by the model 

Risk Potential. The taxon’s risk score (black 

square) is plotted in the risk space. The other 

data represent the risk scores for the 204 

species used to develop and test the PPQ 

WRA model and are plotted for reference. 

The color of these symbols represents the 

known behavior of these species in the United 

States: non-invaders are green, minorinvaders are yellow, and major-invaders are 

red. The symbol used for each species 

corresponds to the outcome of the WRA 

model (Low Risk, Evaluate Further, or High 

Risk). All species that were categorized as 

Evaluate Further (those in the middle region 

of the graph) underwent secondary screening, 
TP E-300, Ver. 2.3 February 11, 2019 11

represent baseline ratings for the plant taxon’s capacity to establish, spread, and cause harm. 

However, consideration of whether an organism can establish in a given climatic region is still a 

fundamental part of pest risk assessment (IPPC, 2013b). For this reason, the PPQ WRA reports 

the geographic potential of a plant taxon separately so that risk managers can make decisions 

appropriate for their jurisdiction (e.g., national, state, local). 

The PPQ WRA addresses geographic potential using three variables—plant hardiness to 

minimum winter temperatures (plant hardiness zones), Köppen-Geiger climate classes, and mean 

annual precipitation bands. For each variable, specific values or ranges of climatic compatibility 

are determined by examining the global distribution of the taxon. These values or ranges are 

evaluated in a series of 36 yes/no questions (see Section 3, Process 4). 

Using geographic information system (GIS) software, we use the information to generate a map 

of the United States where suitable values for each climatic variable jointly occur (Fig. 3). We 

report the map and the percentage of the United States that is climatically suitable for the taxon 

in the WRA. Although all of the United States’ territories are considered when estimating the 

percentage of U.S. area that is suitable, only the map for Puerto Rico is displayed. Work is 

underway to compare the accuracy and precision of this tool to that of other climate-matching 

tools. 

Figure 3. Map of the United States, including Alaska, Puerto Rico, and Hawaii, showing the 

areas estimated as suitable for establishment for the plant taxon under assessment. Map insets for 

Alaska, Hawaii, and Puerto Rico are not to scale.

Entry potential 

If a plant taxon is already present in the United States, or if a permit for its entry has been 

requested, evaluation of this risk element is not necessary. In all other instances, the PPQ WRA 

examines the entry potential of the plant taxon under consideration. This risk element evaluates 

the likelihood that a taxon will enter, whether intentionally or accidentally, as a pathway 

contaminant. The entry potential risk element consists of 12 questions that evaluate the number 

of pathways with which a plant taxon is or may be associated. Taxa that are cultivated elsewhere 

Geographic Potential. Geographic potential is 

determined from the plant taxon’s distribution in 

the world, and is based on three climatic 

variables: plant hardiness to minimum winter 

temperatures, Köppen-Geiger climate classes, 

and mean annual precipitation bands. The area 

shown in red represents the U.S. area where all 

three climatic variables are suitable for the taxon. 

This is typically a conservative estimate, as the 

actual U.S. area suitable is likely to be smaller 

when other limiting variables are considered. 

Furthermore, the area where a species is likely to 

become invasive is likely to be even smaller.
TP E-300, Ver. 2.3 February 11, 2019 12

or positively valued by society are more likely to enter than other less-valued taxa. The risk score 

for this element ranges from 0 to 1, with higher scores indicating a higher likelihood of entry. 

Uncertainty 

Uncertainty is a fundamental component of risk because our knowledge of the factors 

contributing to risk is rarely perfect. Unlike most other WRA systems, in the PPQ WRA 

assessors explicitly categorize uncertainty for every answer and risk element, and we also 

evaluate the sensitivity of the final risk rating to uncertainty. This feature of the PPQ WRA 

provides risk managers and policy-makers additional information for decision-making. 

Summary of uncertainty 

As risk analysts answer the questions in the WRA based on the available evidence, they rate their 

degree of uncertainty for each question. Uncertainty is categorized as negligible (= negl), low, 

moderate (= mod), high, or maximum (= max). Maximum uncertainty is reserved for questions 

that cannot be answered with the available evidence [i.e., those that are answered as unknown 

(?)]. These uncertainty ratings express our degree of confidence in the answers, but also 

simultaneously express our belief that some other answer may be correct. 

The overall level of uncertainty associated with the ES, Imp, and Ent risk elements is also 

summarized using an index of uncertainty that is automatically calculated and reported in the 

workbook. The index ranges from 0 to 1, and reflects the uncertainty rating for each question (as 

categorized by the risk analyst) as well as the relative weight of each question in the risk 

element. During the PPQ WRA model development and validation (Koop et al., 2012), the 

average uncertainty index for the Establishment/Spread and impact risk elements was about 0.17. 

In the final WRA products, the uncertainty index is reported for each risk element. 

Although uncertainty is also categorized for each question in the Geographic Potential risk 

element, currently no index of the overall level of uncertainty is generated for this risk element. 

However, significant sources of uncertainty of this risk element are reported in a written 

summary. 

Uncertainty Analysis 

The sensitivity of a taxon’s risk scores to uncertainty, and therefore to the final conclusion of the 

model, is evaluated using a stochastic (probabilistic) simulation that is conducted with the 

software program @Risk. The stochastic simulation automatically generates new outcomes (i.e., 

new WRA risk scores) by choosing different answers (e.g., no vs. yes) for each question of the 

PPQ WRA. The new answer is selected based on the original answer and its associated level of 

uncertainty. As such, answers with higher uncertainty ratings are more likely to change during 

the simulation than answers with lower uncertainty ratings. For questions that were answered as 

unknown, the stochastic simulation randomly selects an answer based on an equal likelihood of 

occurrence. The process creates new simulated risk assessments and automatically calculates 

their risk scores. 
TP E-300, Ver. 2.3 February 11, 2019 13

The stochastic simulation generates 5,000 sets of simulated risk scores. As demonstrated in 

Figure 4, we graph simulated risk scores in relation to the original risk score for the plant taxon 

under assessment. The distributions for 50, 95, and 99 percent of the simulated scores are shown 

as boxes, while the remaining 1 percent are plotted as outlying points. The original risk score that 

was derived by the analyst is shown as a solid black square. For additional information on 

uncertainty in the PPQ WRA see Section 3, Process 6. 

All simulated risk scores categorized as Evaluate Further (moderate risk) are subjected to 

secondary screening to determine their final risk category. The uncertainty analysis includes an 

inset that lists the percentages of simulations (out of 5,000) corresponding to each of the five 

possible outcomes of the assessment, following secondary screening: 1) High Risk, 2) Evaluate 

Further → High Risk, 3) Evaluate Further → Evaluate Further, 4) Evaluate Further → Low Risk, 

and 5) Low Risk (Fig. 4). 

Figure 4. Sample results of an uncertainty analysis for a plant taxon. Simulated risk scores are 

plotted in the same Establishment/Spread potential (x-axis) and Impact Potential (y-axis) risk 

space as the original risk score that was derived by the risk analyst. 

The larger the distribution of points and boxes around the original risk score, the greater the 

uncertainty in the WRA. Distributions that are located entirely inside one of the three risk 

regions (Low Risk, Evaluate Further, or High Risk) indicate that despite the associated 

uncertainty, the conclusion reached during the PPQ WRA is analytically robust. Distributions of 

simulated risk scores that are centered over the original risk score are “balanced,” indicating that 

the impact of uncertainty has no net direction relative to the original score. Most of the time, the 

center of the distribution will be slightly off-center from the original score, as shown in Figure 4. 

Such a scenario is common and usually not a concern. However, an original risk score that is 

located outside of the three boxes indicates a significant amount of uncertainty associated with 

the assessment. Addressing the uncertainty in the analysis will likely bring the two closer 

together, either by moving the original risk score towards the center of the uncertainty 

distribution (as is usually the case) or coalescing the uncertainty distribution over the original 

Uncertainty analysis. Model simulation results 

(N=5,000) for uncertainty around the risk score 

(filled black square) for a species. The blue “+” 

symbol represents the medians of the simulated 

risk scores. The smallest box contains 50 percent 

of the outcomes, the second 95 percent, and the 

largest 99 percent. The remaining 1 percent of the 

outcomes are shown as outliers outside of the 

boxes. The inset shows the final risk outcomes for 

the simulated risk scores [High Risk, Evaluate 

Further (EF), and Low Risk]. All simulated risk 

scores classified as Evaluate Further undergo 

secondary screening; their final risk classification 

is also shown.
TP E-300, Ver. 2.3 February 11, 2019 14

risk score, or perhaps a little of both. In an ideal WRA, the uncertainty distribution is small, 

centered over the original risk score, and entirely located within one region of risk, and all 

simulated assessments result in the same conclusion as the original assessment. 

The final product 

A summary of the PPQ WRA results, including pertinent graphs and maps, answers to the 

questions, uncertainties, supporting evidence, and general information about the plant taxon, is 

compiled in a Microsoft Word document. The final report is about 15-20 pages long and includes 

the following sections: 

 Title page. Identifies the plant taxon using scientific and common names and provides 

one or more images. 

 Background and species information. Describes the WRA initiating event, the taxon’s 

global distribution, and its status in the United States. 

 Analysis 

o Provides narrative summaries of each of the risk elements assessed 

(Establishment/Spread, Impact, Geographic, and, if applicable, Entry Potential). 

o Reports values for risk scores, uncertainties, and probabilities of invasiveness. 

o Includes figures summarizing the analytical results of the model (geographic 

potential, risk potential compared to that of the taxa used to validate the model, 

and results of the uncertainty analysis). 

 Discussion. Summarizes the final conclusions from the assessment and presents 

additional information relevant to decision-makers. 

 Literature cited. A list of all references and evidence considered. 

 Appendix A. A copy of the data from the Excel worksheet containing the WRA

questions and their answers, uncertainties, scores, and associated evidence. 
TP E-300, Ver. 2.3 February 11, 2019 15

Section 3: Conducting the PPQ WRA 

In this section, we provide guidance to risk analysts on how to conduct a WRA using the PPQ 

WRA process. The processes in this section satisfy the requirements of Stage 1 (Initiation) and 

Stage 2 (Risk Assessment) of the three stages of pest risk analysis described in international and 

regional standards for pest risk analysis: ISPM No. 2 (IPPC, 2011), ISPM No. 11 (IPPC, 2013b), 

and RSPM No. 32 (NAPPO, 2008). 

Before beginning work on a PPQ WRA, ensure you have the following basic documents and 

tools: 

 This guidelines document: Guidelines for the USDA-APHIS-PPQ Weed Risk Assessment 

Process, Version 1 

 The Microsoft Excel workbook file: Form E-300 WRA guidelines and template.xlsx. Save 

this file to an appropriate folder and rename it with the scientific name of the plant taxon 

being assessed (i.e., “Genus species.xlsx”). The majority of your assessment will be 

conducted within this workbook. Here, we refer to this document as the Excel workbook 

or simply as the workbook. The workbook has multiple worksheets, so if we write 

“worksheet,” we are referring to a particular section of the workbook. 

 Word-processing software, such as Microsoft Word. 

 The Microsoft Word document: Form E-301 WRA Document Template.docx. Save this 

file to an appropriate folder and rename it with the scientific name of the plant taxon 

being assessed (i.e., “Genus species.docx”). You will present and summarize the results 

of the WRA in this document. Here, we will refer to this document as the Word template 

or simply as the template. 

 @Risk software. This program is used to evaluate the sensitivity of the risk assessment to 

uncertainty. 

 ArcGIS software. This program is used to generate a map (optional) of the areas in the 

United States suitable for taxon establishment. 

PPQ WRAs are completed using the following eight processes, which are generally conducted in 

the order in which they are listed: 

 Process 1: Gathering background information 

 Process 2: Gathering the evidence 

 Process 3: Answering the questions and describing uncertainty 

 Process 4: Assessing geographic potential 

 Process 5: Assessing entry potential 

 Process 6: Uncertainty analysis 

 Process 7: Finalizing and reviewing the analysis in the Excel Workbook 

 Process 8: Assembling the WRA 

Process 1: Gathering background information 

During this process, familiarize yourself with the taxon being evaluated and begin gathering 

background information that will be needed for the “Species Information” section in the Word 

template. Process 1 includes verifying the identity of the taxon, listing synonyms and common 
TP E-300, Ver. 2.3 February 11, 2019 16

names, and documenting the initiating event for the WRA (i.e., why the WRA is being 

conducted). In this process, we also check online databases to get an overview of the global and 

U.S. distribution of the taxon. To complete this process, follow the guidance in sections 1-1 to 1-

6. 

1-1. Verifying taxon identity and listing synonyms and common names 

Determine the appropriate taxonomic level of analysis. Although species is the most typical 

taxonomic level for analysis, other levels may be more appropriate or necessary, particularly for 

cultivated plants. The use of a taxonomic level above (e.g., genus) or below (e.g., cultivar) the 

species level should be supported by sound rationale. For example, industry may wish to import 

an apparently non-invasive cultivar of a generally recognized invasive species. In this case, the 

WRA should evaluate the taxon at the cultivar level. 

Verify the taxonomic identity to ensure that the scientific name being used is valid. The identity 

of the plant taxon and any synonyms should be determined using the following accepted, 

internationally recognized sources. 

 For taxonomic issues or inconsistencies, search for the scientific name in: 

 Germplasm Resources Information Network (GRIN). United States 

Department of Agriculture, Agricultural Research Service, National Genetic 

Resources Program (NGRP). https://npgsweb.arsgrin.gov/gringlobal/taxon/taxonomysearch.aspx?language=en. Note: We 

typically try to follow taxonomy and nomenclature given by GRIN. 

 The PLANTS Database (PLANTS). United States Department of Agriculture, 

Natural Resources Conservation Service (NRCS), The National Plant Data 

Center. http://plants.usda.gov.

 Kartesz, J. T. The Biota of North America Program (BONAP), North 

American Vascular Flora, Taxonomic Data Center, Chapel Hill, NC. 

http://bonap.net/tdc. 

 If you find inconsistencies or the taxon is not listed in the above sources, sources, 

expand your search to other sources, such as those listed below. If necessary, consider 

contacting taxon-specific experts, or the experts with GRIN, PLANTS, or BONAP. 

 The Plant List, a working list of all plant species. Royal Botanic Gardens, 

Kew and Missouri Botanical Garden http://www.theplantlist.org/. 

 Relevant flora and taxonomic treatments 

 Integrated Taxonomic Information System. http://www.itis.gov/. 

 VASCAN, Database of Canadian vascular plants 

http://data.canadensys.net/vascan/search?lang=en

In the “Species Information” section in the Word template, add the information listed in the 

bullets below. As you conduct the WRA, you will likely revise and amend this information. This 

information may also be entered at the bottom of the “Screening Tool” worksheet in the 

workbook and later transferred to the Word template during Process 7. 

 Taxon, authority, and common name. Record this information at the top of the 

“Species Information” section in the Word template (i.e., the heading). If you use a 

name different than the one provided by GRIN, explain why under “Synonyms.” List 

the primary English common name of the taxon. GRIN will often have numerous 
TP E-300, Ver. 2.3 February 11, 2019 17

common names. If the taxon occurs in the United States, use the most prevalent U.S. 

common name. If it is not present in the United States, use the most representative 

name. The species, authority, and common name will be required elsewhere in the 

Word template (e.g., cover page). 

 Plant family. Plant family is usually straightforward. However, different taxonomic 

systems may treat and organize plant genera among families variably. In such cases, 

we typically follow GRIN taxonomy.

 Synonyms. If the taxon has few synonyms (e.g., less than six), list them all. However, 

if it has numerous synonyms, list only the relevant ones, including the basionym (the 

original, validly published name of the plant taxon). Relevant synonyms are those that 

would be encountered or used today or in the recent past, or those that were used in 

the evidence you provided in the WRA. If the plant taxon is currently being referred 

to by multiple scientific names in the literature, identify the names and describe who 

is using those synonyms. Indicate whether or not you used these additional synonyms 

in the literature search. 

 Common name(s). Only list the major English common names and any other names 

that were relevant for the WRA. After each name, provide at least one citation 

documenting the use of that name. 

1-2. Familiarize yourself with the plant taxon 

Before beginning the main literature review for the WRA, spend a few minutes familiarizing 

yourself with the taxon. Consider the taxon’s size, life form (herb, vine, tree, grass, etc.), life 

cycle (annual, perennial), and preferred natural habitats. Find this information through a general 

web search (e.g., Google or Bing), taxonomic treatments, or species factsheets. We also 

recommend searching for images of the taxon on the internet. A basic understanding of the taxon 

will help you evaluate the evidence. 

1-3. Documenting the initiating event 

Under “Initiation” in the “Species Information” section of the Word template, describe in a few 

sentences why the WRA is being conducted. Cite emails and/or discussions that resulted in the 

initiation. Include the name and affiliation of the requestor. 

For examples of why a PPQ WRA may be initiated, see Section 1 (under “Risk analysis 

framework”) above. 

1-4. Briefly review the foreign and U.S. distribution of the taxon 

Understanding the foreign and U.S. distribution of a taxon is important for risk managers who 

must make regulatory or operational decisions, but it is also important for risk analysts 

conducting the assessment. Taxa that are present in the United States will require additional 

evaluation to determine the extent and nature of their U.S. distribution. Information required for 

this circumstance is best obtained while conducting the risk assessment and is described under 

Process 2. 

During Process 1, it is sufficient to review a species’ distribution by visiting a few online 

databases. This information will help prepare the analyst for the depth of evaluation that will be 

needed under Process 2. 
TP E-300, Ver. 2.3 February 11, 2019 18

Briefly review the species’ distribution by visiting the GRIN, PLANTS, and BONAP databases 

identified in section 1-1. In addition, visit the Global Biodiversity Information Facility (GBIF), 

which maintains records on plant distribution. For records associated with global positioning 

system coordinates, GBIF maps occurrences (e.g., Figure 5). During Process 4 (Assessing the 

geographic potential), the analyst will download the distribution records from GBIF. 

Figure 5. GBIF map of the plant records associated with point occurrences for Heliotropium 

europaeum.

Process 2: Gathering the evidence 

Although PPQ WRAs are summarized in a narrative format, the assessment itself is highly 

structured and uses a set of specific questions to evaluate risk. Specific evidence is required to 

answer the questions in the PPQ WRA. In Process 2, we gather the evidence needed to answer 

these questions, as well as to describe the taxon’s U.S. distribution and status. During this 

process, we may also discover additional information relevant to the taxonomy and foreign 

distribution of the species. 

We recommend that new analysts begin with section 2-1. Experienced analysts already familiar 

with the PPQ WRA may begin with section 2-2. 

2-1. Familiarizing yourself with the questions and the Excel workbook 

Before starting your first PPQ WRA, we recommend that analysts review the WRA questions 

and their associated question-specific guidance. An understanding of the questions and the kind 

of evidence required to answer them is critical for compiling the most useful evidence and for 

completing the assessment in a timely manner. This also helps reduce analyst subjectivity. A list 
TP E-300, Ver. 2.3 February 11, 2019 19

of the questions and their associated guidance is provided in Appendices E through G. The 

questions and their guidance are also found in the “Screening Tool” worksheet of the workbook, 

where the guidance for each question is embedded as a comment; hover your mouse over the cell 

containing the question to see a pop-up with the guidance. 

The question-specific guidance provides a detailed description of the kind of evidence required 

to answer each question. It describes the evidence required for yes and no responses, and 

indicates when a question can be answered as no based on a lack of evidence or when using 

congeneric information would be inappropriate. The guidance also includes definitions, 

clarification of complex issues, lists of recommended resources, suggestions for search criteria, 

and full examples of answered questions. There is a large amount of information in the questionspecific guidance, so it is important to carefully read it before you begin working on a WRA. 

Analysts usually become very comfortable with the material only after completing 3-5 

assessments. 

We also recommend that all new analysts familiarize themselves with the content and structure 

of the workbook. In Table 1 we list all of the worksheets in the workbook and briefly describe 

their content or purpose. 

Table 1. Descriptions of worksheets within the PPQ WRA Excel workbook. 

Worksheet Description 

Revision History The WRA process is managed under a quality management system 

to ensure that all products are of the highest quality and are based on 

sound science. Under the quality management system, some 

documents are controlled and must be approved by the appropriate 

staff member before they can be used. The workbook is a controlled 

document; its version, effective date, and approver are recorded 

here. This worksheet also provides a history of the revisions to the 

workbook. 

Screening Tool All of the questions in the WRA and their associated guidance are 

located in this worksheet. Columns for recording the evidence, 

answer, and uncertainty for each question are provided (see 

Processes 2 and 3). Near the bottom of the worksheet are cells 

reporting risk scores, probabilities of invasiveness, and overall 

model results. A block of gray cells is also provided for the analyst 

to record general information about the taxon, the assessment, and 

his or her name and affiliation. The references used to support the 

WRA are recorded below that block of cells. 

Risk Score Plot This worksheet contains two graphs that we use in our WRA 

products. The top graph (the risk score plot) is automatically 

updated as the analyst answers questions in the “Screening Tool” 

worksheet. The bottom graph contains the results from uncertainty 

analysis; it is automatically populated when the analyst enters the 

5,000 simulated risk scores from the stochastic simulation (see

Process 6). 
TP E-300, Ver. 2.3 February 11, 2019 20

Worksheet Description 

Uncertainty This worksheet contains the stochastic model used by the software 

program @Risk for the uncertainty analysis (see Process 6). 

Analysts do not need to enter or change any information in this

worksheet. It is password protected to avoid accidental changes to 

its content. If you do not have @Risk installed and open on your 

computer, many of the cells on this worksheet will report errors. 

This is normal and can be ignored. 

LUT This is the Look-Up Table (LUT) for determining what score to 

assign to answer choices. Analysts do not need to enter or change 

anything in this worksheet. It is also password protected. 

Notes Analysts can record additional information on this worksheet that 

does not correspond to any of the specific questions on the 

“Screening Tool” worksheet. 

Climate Worksheet This is a blank worksheet that the analyst can print and used for 

taking notes during analysis of a taxon’s geographical potential 

(Process 4). 

GBIF Data Data downloaded from the GBIF database is recorded in this 

worksheet (Process 4). 

Maps This worksheet records a few pieces of information related to the 

geographical potential analysis. In addition, the analyst inserts a 

copy of the final map showing the regions of the United States that 

are suitable for the taxon’s establishment (Process 4). 

TableForWord PPQ WRAs are conducted in the Microsoft Excel workbook but are 

reported in a Word document. This worksheet automatically 

duplicates the information from the Screening Tool worksheet into a 

format that the analyst can paste into Word (Process 7). 

2-2. General approach to evidence gathering 

We recommend gathering evidence source by source and not question by question. In other 

words, as you read each source and find information relevant to a particular question, record that 

evidence in the “Notes (and references)” column of the “Screening Tool” worksheet of the 

workbook. Consider quoting from the source rather than paraphrasing when the source is 

particularly compelling or useful. Try to read each source only once. 

Focus on gathering sources that provide direct evidence. Direct evidence answers the question 

without the need for any indirect or circumstantial evidence, inferences, or assumptions. For 

example, for the question “Is the taxon shade adapted?” evidence stating “This species is adapted 

to shade and can survive under low light conditions for extended periods” is considered direct 

evidence. In contrast, evidence stating “This taxon is a pioneer and early successional species” is 

not direct evidence, but could be interpreted to suggest that the taxon is not shade adapted 

because pioneer and early successional species are generally light-demanding taxa. This latter 

type of evidence, while valid, is indirect or circumstantial, and usually associated with higher 

uncertainty. 
TP E-300, Ver. 2.3 February 11, 2019 21

During Process 2, enter only relevant information in the “Notes (and references)” column for 

each question. Do not answer the question or categorize the uncertainty until you have completed 

the majority of the literature search or have difficulty finding relevant new information. When 

you have accumulated enough evidence to inform a particular question, answer it and categorize 

the level of uncertainty in the “Answer” and “Uncertainty” columns of the “Screening Tool” 

worksheet as described under Process 3. 

2-3. Reviewing the literature 

General literature search. Consult the types of resources listed below (in whatever order you 

choose) to gather the majority of the evidence for the plant taxon being assessed. Although we 

list them first, note that much of the world’s knowledge on invasive species is not published in 

peer-reviewed journals. Gather evidence from different sources and locations where the taxon 

occurs. Include information on impacts and invasive behavior from anywhere in the world, 

including information from the WRA area if it occurs there. In general, try to find two to four 

references for each question; reduce uncertainty by using more sources if the evidence is weak or 

has discrepancies. A broad evidence base should strengthen the assessment and lower your 

uncertainty. 

 Major electronic journal databases and search engines (e.g., CAB Abstracts, BIOSIS, 

Web of Science, AGRICOLA, SCOPUS). These can be used to find primary literature 

(e.g., research articles) on the taxon. The introduction/background and methods sections 

of articles that do not initially appear relevant often provide useful information on 

biology and invasive species status. 

 Major weed/invasive species books and guides (e.g., Holm et al., 1997; Weber, 2003). 

These works often summarize pest biology, impacts, management strategies, and ongoing 

efforts on control. 

 Proceedings (e.g., Brunel, 2005; Veitch et al., 2011). These are collections of seminars 

and posters presented at meetings, workshops, and conferences. They include general and 

taxon-specific information on research, policy, and management information. 

Proceedings are usually available online. 

 Pest databases and other factsheets (e.g., CABI, 2013a, 2013b; ISSG, 2013). 

 General references (books, textbooks) on biological invasions or weeds (e.g., Booth et 

al., 2003; Radosevich and Holt, 1984). Although these infrequently provide detailed 

taxon-specific accounts, they sometimes contain important information. 

 Regional plant lists (e.g., Acevedo-Rodríguez and Strong, 2012; Space and Flynn, 

2002). These lists of native and/or exotic taxa occurring in a region are useful for 

determining presence/absence of a plant taxon in a region or country and, for some types 

of questions, are sometimes the only references available. 

 Annotated plant lists (e.g., Randall, 2007; Verloove, 2006; Waterhouse, 1993). These

can provide additional information, such as taxon status (casual, naturalized, invasive), 

date of introduction or naturalization, distribution within a region, pathways/vectors, life 

form, and habitats invaded (natural, agricultural, etc.). 

 Floras (e.g., eFloras, 2013). These are botanical treatments of native and naturalized taxa 

in a region. They usually include dichotomous keys for identification, detailed botanical 

descriptions, and information on status, habitats, distribution, and whether a taxon is 

native or naturalized. They can be useful for determining plant life form, taxonomy, 
TP E-300, Ver. 2.3 February 11, 2019 22

family, and fruit/seed morphology. Floras usually provide information needed for the 

general botanical description of the taxon under the “Species Information” section of the 

WRA template. 

 Herbaria (e.g., Univ. of California, 2014; Univ. of Florida, 2014). These are collections 

of pressed and dried plants that are often labeled with information from the collection 

sites. Because herbarium records often precede published distribution information by 

years to decades, they are useful for identifying additional regions of occurrence and for 

evaluating status in an area (e.g., casual, naturalized, invasive) and possible impacts. 

 Gardening compendia (e.g., Bailey and Bailey, 1976; Foulis, 2001). These provide 

horticultural information and may help determine taxon popularity and date of 

introduction. 

 Gardening websites and forums (e.g., Dave's Garden, 2014; GardenWeb, 2014). These 

sites can contain plant horticultural factsheets, which give information on plant behavior 

and general growing conditions (light/shade, soil, plant hardiness zones). They are useful 

for determining cultivation status and for finding evidence of anthropogenic impacts and 

control. 

 Google Scholar (http://scholar.google.com). This search engine focuses on scientific 

literature, including peer-reviewed articles, proceedings, and reports. It can provide very 

useful and timely information. Results may overlap with electronic journal database 

results, but Google Scholar is useful for finding more recent publications and reports that 

are not available in other databases. Links to full-text PDF documents are sometimes 

available. 

 General internet search engines (e.g., Google, Bing). This type of search is good for 

finding information from local or state weed managers, university extension agencies, 

and non-scientific sites (e.g., forums, gardening sites, invasive species sites). 

 Other WRAs (e.g., Univ. of Hawaii, 2014; DEPI, 2014). WRAs prepared by other 

governments or groups can be very useful to fill in gaps and verify other information. 

If you need more evidence to answer the questions after conducting a general literature search as 

described above, conduct more specific searches or search at the genus level. 

Specific searches. Conduct question-specific searches using the recommended resources and 

keywords for a given question (e.g., taxon name + breeding system + mating system). These 

resources and keywords are available within the question-specific guidance of this document (see 

Appendices E through G). However, we do not recommend searching for more than an hour to 

fill in gaps in information after the general literature search, because such information may not 

be available and it may not alter the outcome of the WRA. Questions can always be answered as 

unknown or no (based on a lack of evidence) as appropriate. If, after completing the WRA and 

running the uncertainty simulation, it becomes apparent that additional evidence would make a 

difference in the outcome of the WRA, then consider further, targeted searches. 

Congeneric searches. We can use information from closely related taxa (i.e., congeners) to fill in 

some information gaps, particularly for “simple” biological traits that are relatively well 

conserved within taxonomic groups. For example, fruit type is generally well conserved within 

plant genera, making it likely that most members of a given genus will have the same fruit type 

and dispersal mechanisms. However, we advise against using congeneric evidence for questions 
TP E-300, Ver. 2.3 February 11, 2019 23

about more complex processes. For example, congeneric evidence should not be used to answer 

questions about a taxon’s invasiveness, weediness, impact, and other “behaviors,” as they are 

determined by complex interactions of multiple factors (e.g., species traits, environmental 

features, and human processes). In the question-specific guidance (Appendices E, F, G, and H), 

we clarify for which questions it is appropriate to use congeneric information. As a general rule, 

we try to limit the use of congeneric evidence to only a few questions in any given WRA, 

because the assessment should focus on the specific taxon. 

2-4. Gathering evidence on U.S. distribution and status of the plant taxon 

As you gather evidence for the WRA questions, document information relevant to the U.S. 

distribution and status of the taxon. Specifically, determine if the taxon is naturalized or 

cultivated in the United States. If it is naturalized, describe that distribution and any efforts to 

control or manage the species. Although that information does not affect the outcome of the 

assessment (i.e., the risk score and rating), it is part of the background information on the taxon 

and is likely to be critical information for decision-makers when setting policies or prioritizing 

resources for weed management. Specific issues that should be addressed and suggested 

resources for addressing them include: 

 Extent of U.S. cultivation. Is the plant taxon cultivated in the United States, and if so, 

what is the extent of cultivation? Decision-makers consider the extent of cultivation when 

deciding whether or not a taxon should be regulated, because regulatory programs are 

likely to be ineffective against widely cultivated or distributed taxa. The extent of 

cultivation varies widely and can be evaluated in several ways (Fig. 6). If the taxon is 

cultivated in the United States, consider whether it is cultivated commercially or only 

privately. Taxa that are offered in major retail stores (e.g., Lowe’s, Home Depot, 

Walmart) or grown and distributed by major commercial growers (see below) can be 

considered widely cultivated. Also consider visiting online gardening forums and seed 

exchange websites to gauge the amount of interest and private trading. Finally, consider 

the number of cultivars that are available for the taxon, which can indicate how 

extensively it is cultivated. Examples of sources for this information include: 

o Major commercial growers, distributors, and retail stores: 

 Monrovia: http://www.monrovia.com/plant-catalog/

 Green Leaf: 

http://greenleafnursery.com/index.cfm/fuseaction/plants.main/index.htm

 Bailey Nurseries: http://www.baileynurseries.com/imagelibrary/gallery/

 San Marcos Growers: http://www.smgrowers.com/index.asp 

 Lowe’s Home Improvement: 

http://www.lowes.com/cd_Lowes+Plant+Guide_253427968_?url=plantse

arch.aspx

o National and regional plant finder databases are good sources to determine if a 

given taxon is commercially propagated at regional or local levels. 

 Backyard Gardener: http://www.backyardgardener.com/Plant-Index/

 Plant Information Online: https://plantinfo.umn.edu/ 

 Florida and U.S. Gulf Coast: http://www.plantfinder.com/

o Major U.S. and North American horticultural references are useful as well. 
TP E-300, Ver. 2.3 February 11, 2019 24

 Hortus [e.g., Bailey, L. H., and E. Z. Bailey. 1976. Hortus Third: A 

Concise Dictionary of Plants Cultivated in the United States and Canada 

(revised and expanded by The Staff of the Liberty Hyde Bailey 

Hortorium). Cornell University. 1290 pp.] (Compare listings among the 

different editions to determine whether a taxon was introduced recently.) 

 Dirr, M. 1998. Manual of Woody Landscape Plants: Their Identification, 

Ornamental Characteristics, Culture, Propagation and Uses. Stipes Pub., 

Champaign, Illinois. 1187 pp. 

 Online Plant Guide: http://www.onlineplantguide.com/Index.aspx 

o Online Gardening Forums 

 Dave’s Garden: http://davesgarden.com/guides/pf/ 

 GardenWeb: http://forums.gardenweb.com/forums/

o Garden Collections 

 Missouri Botanical Garden: 

http://www.livingcollections.org/mobot/Home.aspx

 Fairchild Tropical Garden: http://www.fairchildgarden.org/scienceconservation-/plant-collections/tropical-fruit-collection/list-of-livingplants

 The Arnold Arboretum: http://arboretum.harvard.edu/plants/plantinventory/ 

o General internet search engines (e.g., Google, Bing): You can also do a general 

internet search with the keyword “buy” + the taxon name to generate a list of 

places selling the plant online. 

 Naturalized distribution in the United States. Is the taxon naturalized in the United 

States, and if so, what is the extent of its distribution and invasive status? In how many 

and which states is it naturalized? If only a few occurrences exist, you should verify if the 

plant taxon is truly naturalized or if the occurrences represent escaped or cultivated 

plants. Recommended sources to address this issue include the following: 

 Kartesz, J. T. The Biota of North America Program (BONAP). North 

American Plant Atlas, Taxonomic Data Center, Chapel Hill, North Carolina. 

http://bonap.net/tdc. 

 The PLANTS Database. United States Department of Agriculture, Natural 

Resources Conservation Service (NRCS), The National Plant Data Center. 

http://plants.usda.gov. 

 EDDMapS: Early Detection and Distribution Mapping System. The 

University of Georgia, Center for Invasive Species and Ecosystem Health. 

http://www.eddmaps.org/. Not all observations in EDDMapS have been 

verified, which raises the uncertainty surrounding the data. 

 Management and regulatory status in the United States. Is the plant taxon being 

managed or regulated in the United States, and if so, what is being done? The scope of 

this evaluation is very broad and includes action taken by any state, private, or public 

entity, including private landowners and other Federal agencies. For instance, do any 

states list the taxon as a Noxious Weed? Is it on advisory lists or targeted for control by 

Federal agencies, such as the U.S. Forest Service, the U.S. Fish and Wildlife Service, or 

the Department of Transportation? Likewise, is it on advisory lists or targeted for control 

by groups such as invasive plant or exotic pest plant councils? We generally do not 
TP E-300, Ver. 2.3 February 11, 2019 25

contact stakeholders and experts for this type of information, but that may be warranted if 

you find conflicting information in the literature. 

Some preliminary sources for this information include those listed below. Other sources 

that provide more detailed information will likely appear during the literature review. 

o PPQ. 2010. Federal Noxious Weed List 

(http://www.aphis.usda.gov/plant_health/plant_pest_info/weeds/downloads/weedl

ist.pdf). United States Department of Agriculture, Animal and Plant Health 

Inspection Service, Plant Protection and Quarantine (PPQ). Normally, the taxon 

being assessed would not already be listed as a Federal Noxious Weed, but we 

listed this resource here in the rare case that you need to consult it. 

o The BONAP and PLANTS databases linked above. However, these databases 

may not have the most up-to-date information on state weed lists, and they 

sometimes confound noxious weed lists with noxious seed lists. 

o The National Plant Board’s page on Regulated Noxious Weeds 

(http://nationalplantboard.org/laws-and-regulations/state-regulated-noxiousweeds/). This page contains links to state and federally regulated weeds. 

o State Noxious-Weed Seed Requirements Recognized in the Administration of the 

Federal Seed Act 

(http://www.ams.usda.gov/AMSv1.0/getfile?dDocName=STELPRDC5090172). 

United States Department of Agriculture, Agricultural Marketing Service. 

o Least Wanted: Alien Plant Invaders of Natural Areas 

(http://www.nps.gov/plants/alien/fact.htm). United States Department of the 

Interior, National Park Service, Plant Conservation Alliance, Alien Plant Working 

Group. 

o Invasive Plant Atlas of the United States 

(http://www.invasive.org/weedus/index.html). Collaborative project between the 

National Park Service, the University of Georgia Center for Invasive Species and 

Ecosystem Health, the Invasive Plant Atlas of New England and the Lady Bird 

Johnson Wildflower Center. 

o National Association of Exotic Pest Plant Councils (EPPCs) 

(http://www.naeppc.org/). This organization is a coalition of state and regional 

Exotic Pest Plant Councils (EPPCs) and Invasive Plant Councils (IPCs). This site 

has links to the websites for U.S. EPPCs and IPCs. 
TP E-300, Ver. 2.3 February 11, 2019 26

Extent of 

Cultivation 

Level of Cultivation Public Interest in 

gardening forums 

Number of 

Cultivars 

Widely 

Cultivated 

Commercial 

Sold by major retail stores such as 

Walmart, Lowe’s, Home Depot 

Dozens of 

comments Dozens or more 

Grown by major plant distributors 

such as Monrovia, Green Leaf, 

Bailey, San Marcos Growers 

 

Offered by only local, small, or 

specialized nurseries Some comments Some cultivars 

Private 

Maintained by only specialty or 

private collectors 

Limited 

Cultivation Only present in botanical gardens A few A few 

Not Cultivated Not cultivated No public 

comments None 

Figure 6. Framework for considering the extent of a taxon’s cultivation in the United States. 

Process 3: Answering the questions and describing Uncertainty 

Most of the evidence gathered during Process 2 will pertain to the WRA questions contained in 

the “Screening Tool” worksheet in the workbook. In Process 3, we use this evidence to answer 

the WRA questions and qualitatively describe the uncertainty associated with the answers. 

Although we present general guidance for this process in three separate sections below, risk 

analysts will usually be completing all of these sections concurrently. You should familiarize 

yourself with all the guidance below before starting this process. For further information related 

to each question, refer to the question-specific guidance (Appendices E, F, G, and H). 

3-1. Presenting and evaluating the evidence 

In this section, we provide general guidance on how to organize, present, and evaluate the 

evidence so that your answers and uncertainty ratings are well and clearly supported. The 

gathered evidence should be placed in the “Notes (and references)” column of the “Screening 

Tool” worksheet in the Excel workbook. Because the text in this column will form part of the 

final Word document, present the evidence in complete sentences, organized in an intuitive way, 

and properly cited. Review the evidence and ensure that it is assigned to the appropriate 

question. 

Present the strongest evidence first. Present the strongest evidence first, unless the nature of the 

question or the evidence suggests some other order is more appropriate. Direct evidence is 

stronger than indirect evidence. If you have strong evidence from multiple sources and 

uncertainty is negligible, consider deleting weaker evidence to keep the notes succinct. 

Use direct quotations or paraphrase, as appropriate. For expediency, we sometimes present the 

evidence by quoting from the literature. This has the added benefit of capturing the source’s 

wording. However, when you need to report evidence from many different sources, paraphrasing 
TP E-300, Ver. 2.3 February 11, 2019 27

may be more concise and understandable. In such instances, citing multiple sources in a single 

sentence is good practice. 

Provide citations for all evidence. Cite all published literature and information from databases, 

webpages, government documents, and personal communications. Citations help ensure 

transparency and allow readers to locate the original source if needed. 

Address each question as precisely as possible. Eliminate information that is not directly related 

to the question to keep the evidence as succinct as possible and to avoid confusing the reader. 

For example, the evidence “Taxon X reduces species diversity, lowers crop yield, and hinders 

navigation” should be split up among the appropriate questions. Thinking critically about what 

the different questions are asking is important. Although some questions are related or build on 

each other, you should not use the same evidence to answer multiple questions, because each one 

addresses a different risk factor. If the distinction between different questions is not apparent to 

you, carefully read the guidance or consult with the team. 

Limit the number of inferences. The PPQ WRA is an evidence-based process, meaning that the 

evidence answers the questions, not the analyst. If direct evidence is not available to support an 

answer, the analyst can make inferences using indirect and circumstantial evidence. An inference 

is a conclusion drawn by connecting indirect pieces of evidence in a logical manner. However, 

analysts should limit the number of inferences as overusing them will weaken the assessment and 

may have a significant impact on the final risk score of the species. In general, we recommend 

limiting the use of inferences to those questions that have the word “likely” in them (i.e., ES-15, 

ES-16, Imp-N4, Imp-N5, and Imp-P3), but inferences can be used elsewhere as appropriate. 

When you make an inference, be sure to make it clear that this is the case (e.g., state that the 

evidence suggests X because of Y and Z...). 

Do not use speculation or suppositions to support positive answers. A supposition is an opinion 

or statement based on incomplete evidence. For example, for the question “Does the taxon 

change species diversity?,” answering yes would be inappropriate if we only knew that the taxon 

being assessed is a vine and that some other species of vines have reduced biodiversity. In this 

case, the analyst would be speculating, because not all vine species reduce biodiversity. When 

reasonable, you can use speculation to justify an answer of “unknown” when a lack of evidence 

would otherwise suggest a response of no (see additional guidance below). For example, the 

following would be appropriate: “Although we found no direct evidence that taxon X reduces 

biodiversity, this taxon is spreading aggressively into undisturbed forests [citation]. Because it is 

very similar to its congeners Y and Z, which have been shown to reduce biodiversity 

[citation(s)], we answered this question as unknown.” Compare this example with the one given 

at the beginning of the paragraph. 

Provide an explanation for your answer when the evidence is conflicting. Sometimes the 

evidence is conflicting because it supports different answers. In these cases, organizing and 

clearly presenting all of the evidence is important. When evaluating each piece of evidence, 

consider its overall weight. Consider the evidence’s quality, source, independence, and 

specificity to the taxon and question at hand, as well as how many pieces of evidence support 

one answer versus another. The evidence may support a single, best answer, but if not—if it 
TP E-300, Ver. 2.3 February 11, 2019 28

equally supports two or more choices—answer the question as unknown. Regardless of the 

outcome, provide an explanation for the choice and the associated uncertainty. 

Do not over-interpret any given piece of evidence. It can be easy to read too much into any piece 

of evidence or to overuse it for multiple questions. For example, a regional list of invasive taxa 

may classify a taxon as a “transformer.” Based on the definition of that term in the source, the 

analyst may be tempted to use the information to answer questions about impacts on ecosystem 

processes, habitat structure, and species diversity. However, unless the source specifically 

mentions that the taxon had all of those impacts, this would be over-interpreting the evidence. In 

this case, we only know for certain that the taxon had at least one of those impacts. The analyst 

should not ignore the evidence that the taxon is considered a “transformer,” but will have to 

decide how best to use that evidence (e.g., for a question for which much less other relevant 

information was found). For the other questions where the evidence could also apply, it would be 

appropriate to state, “This taxon is considered a transformer, but it was not clear what specific 

impacts it is having. Consequently we answered unknown for this question about changes to 

ecosystem processes, but assumed that the taxon is changing biodiversity (see Imp-N3).” 

Use evidence relating to weeds and invasive plants with caution. As explained in the 

Introduction, these terms are inconsistently defined and used. Within the scope of answering the 

assessment questions, we follow Richardson et al.’s (2000) concept of an invasive species as one 

that can establish, naturalize, and spread, regardless of the type of system in which it is present. 

This definition does not consider harm the taxon may be causing. A weed, on the other hand, has 

negative impacts or is perceived as problematic. Its ability to naturalize and spread is not relevant 

for assessing impact. Thus, within the scope of the WRA questions, these terms are distinct and 

address only one of the risk elements each: invasiveness gives information about 

Establishment/Spread Potential, while weediness gives information about Impact Potential 

(Table 2). When compiling and evaluating the available evidence, carefully consider how each 

source uses the terms “weed” and “invasive plant.” As a general rule, the analyst should not use 

the term “weed” in evidence presented under Establishment/Spread Potential or the term 

“invasive plant” in evidence presented under Impact Potential. 

Table 2. Comparison of the terms “invasive plant” and “weed” as they relate to the PPQ WRA. 

Term Definition Relevant WRA Section 

“Invasive plant” A plant capable of establishing, 

naturalizing, and spreading, regardless of 

the type of system in which it is present 

Establishment/Spread 

Potential 

“Weed” A plant taxon that has negative impacts or 

is perceived to be problematic by people 

Impact Potential 

Indicate the use of congeneric data. A congener is a species that is in the same genus as the 

taxon being assessed. In the PPQ WRA, data from congeners with similar life forms and biology 

can be used to support weak or incomplete evidence from the taxon being evaluated. Congeneric 

data can also be used when specific data in unavailable. When using such information, the 

source(s) must identify the congener(s), and the evidence must be clear and reasonably certain. 

Statements/facts about an entire genus (e.g., “the genus Vaccinium produces fleshy fruits that are 
TP E-300, Ver. 2.3 February 11, 2019 29

dispersed by birds”) are not considered congeneric information and may be used as direct 

evidence for every taxon within the genus. 

Limit the use of congeneric data to answer Impact Potential questions. Generally, congeneric 

information can be used to support a yes or no answer for Establishment/Spread Potential risk 

element questions, because many of those questions concern relatively simple biological traits 

that may be well conserved among members of a genus (e.g., fruit type, life form). In contrast, 

Impact Potential questions focus on the consequences of complex interactions of traits and the 

invaded environment. Therefore, analysts should not use evidence from congeners to support yes 

answers to Impact Potential questions for the species being evaluated. However, you can use 

congeneric evidence to support answers of no and unknown when reasonable. For example, if the 

taxon is not well known and you think it could have certain impacts based on its similarity to 

congeners that have these impacts, answer unknown and provide your reasoning. Use of 

congeneric information is allowed in this fashion in the Impact Potential risk element, because in 

this element answers of no and unknown are scored as 0. We discuss a few other exceptions to 

the use of congeneric information for the Impact Potential risk element in the question-specific 

guidance (Appendix F). 

3-2. Answering the questions 

After you have evaluated the evidence for a given question and have presented and organized it 

succinctly, you are ready to answer. Enter your answer (i.e., y, n, ?, a, b, c, d, e, or f) in the 

“Answer” column of the “Screening Tool” worksheet. We recommend that you enter answers 

using the keyboard, but you can also select them using the dropdown menu within each answer 

cell. All answers must be entered exactly as specified and in lowercase without periods. Do not 

cut and paste answers from one question to another, as that can alter formulas associated with 

specific cells. Once you enter an answer, the spreadsheet displays the risk score associated with 

that question in the “Score” column. The spreadsheet also maintains a running sum of the risk 

scores for each risk element in the “Risk Score Summary” section near the bottom of the 

“Screening Tool” worksheet (cells E127 through E134; Fig. 7). 
TP E-300, Ver. 2.3 February 11, 2019 30

Figure 7. Example of the “Risk Score Summary” section (cells A126-E134) of the “Screening 

Tool” worksheet. 

Based on the available evidence, one of four scenarios will emerge for each answer. We discuss 

these below, as well as particular concerns with multiple-choice questions. 

Answering based on good evidence. In this scenario, direct or indirect evidence sufficiently 

supports a given response, and the answer should be clear. 

Answering when conflicting evidence exists. You found direct/indirect evidence, but it is 

conflicting, and choosing a particular answer is difficult. In these cases, answering unknown is 

appropriate. Sometimes, though, the conflicting information represents variation in taxon 

biology, where some varieties or populations possess a trait and others do not (e.g., selfcompatibility). In such cases, answering unknown is reasonable, but so may be answering yes or 

no depending on whether you are taking a conservative or liberal approach. The important thing 

to do is to explain your approach in the “Notes (and references)” column. 

The other two scenarios involve a lack of evidence, and the analyst must answer either no or 

unknown. Below we provide some general guidance for these two scenarios, but when no 

information is found, see Appendices E through G for detailed guidance for each question. 

Answering no based on a lack of evidence. When the plant trait or behavior being evaluated is 

readily observable and the taxon is relatively well known, answering no based on a lack of 

evidence may be appropriate. This approach is based on the assumption that if the taxon had a 

given trait or caused some kind of impact, it would have been reported in the literature. For 

example, although no source specifically states that daffodils are not parasitic or that they have 

been tested and found not to be parasitic, you can reasonably answer that they are not parasitic 

because daffodils are relatively well studied and well known. The same reasoning applies to 
TP E-300, Ver. 2.3 February 11, 2019 31

other questions, including those in the Impact Potential risk element. For example, consider a 

well-known, widely distributed, woody vine taxon that is only established in natural systems. 

Questions relating to impact in agricultural systems could be answered as no with low 

uncertainty because for a well-known taxon such impacts would likely have already been 

reported if they were valid. As the knowledge base about a taxon decreases, you should use 

higher levels of uncertainty with these types of answers until, ultimately, answering unknown 

would be more appropriate. 

Answering unknown based on a lack of evidence. When no or little information has been found 

and the plant trait being evaluated is not readily observable, or the taxon is not well studied or 

well known, answering unknown is likely most appropriate. For example, for the question “Is the 

plant self-compatible?” an analyst should answer unknown when no information has been found 

because that trait is not readily observed or easily tested. A similar approach should be taken 

when the taxon is new to science, is generally not well known or well studied, or has never or 

only recently been introduced outside of its native range. One practical consequence of 

answering unknown is that all possible answers for that question are considered equally likely 

(which affects the uncertainty analysis). In the question-specific guidance (Appendices E, F, G, 

and H), we identify those questions for which a response of unknown is appropriate when 

information is unavailable. 

Questions with multiple-choice answers. Five questions in the PPQ WRA require the analyst to 

choose among multiple-choice answers (Table 3). Except for ES-13, these questions relate to the 

behavior of a taxon elsewhere (ES-1) or how it is perceived by people (Imp-N6, Imp-A4, and 

Imp-P6). Choose the best answer from the available evidence. Because analysts can usually find 

information that rules out one or more possible answers, we think answering unknown is not 

ideal for these questions. However, answering unknown may be appropriate when the taxon is a 

new organism without a long history in either cultivation or the wild. This includes taxa that 

recently formed naturally through hybridization or polyploidization, or were artificially created 

by traditional breeding or other modern techniques. Answering unknown is appropriate for those 

taxa because they have not yet had an opportunity to behave in an invasive or weedy manner. 

Importantly, for your answer to multiple-choice questions, you must enter two alternate choices 

in the “Screening Tool” worksheet (see cells I27-K33). These alternate choices are used in the 

uncertainty analysis (Process 6). For the “2nd choice,” choose the second most likely answer, and 

choose the third most likely answer for the “3rd choice.” For any given question, the two alternate 

choices must be different from the original answer, but they do not have to be different from 

each other; if no other option makes sense as the “3rd choice,” use the same answer as that for 

“2nd choice.” At the end of the evidence that is presented in the “Notes (and references)” column 

for multiple-choice questions, list the alternate choices so that this information is carried over 

into the Word document (e.g., The alternate choices for the stochastic simulation were “d” and 

“e”). 
TP E-300, Ver. 2.3 February 11, 2019 32

Table 3. Multiple-choice questions in the PPQ WRA. 

Risk Element Question Possible Answers 

ES-1 (Establishment/Spread status outside its native range) a to f 

ES-13 (Minimum generation time) a to d 

Imp-N6 (Environmental/natural areas weed) a to c 

Imp-A4 (Weed of anthropogenic systems) a to c 

Imp-P6 (Weed of production systems) a to c 

3-3. Determining uncertainty ratings for questions 

Uncertainty is a fundamental part of pest risk analysis because our knowledge of the factors 

contributing to risk is rarely complete or clear. This applies to the evidence available to evaluate 

the risk factors, but also to the risk models themselves, which may not consider all of the factors 

associated with risk. In the former case, sources of uncertainty include missing or incomplete 

information, inconsistent or conflicting data, and dated or erroneous information. Because of the 

quantity and quality of the available evidence, risk analysts are not always completely confident 

in the answers they choose. For the sake of transparency, risk analysts are compelled to report 

uncertainty in pest risk assessments (IPPC, 2011, 2013b) and consider how it may affect the 

results of the assessment. 

In the PPQ WRA, we consider uncertainty at three different levels. First, we report the 

uncertainty associated with each answer. Second, we calculate static estimates of the overall 

levels of uncertainty associated with each risk element. Finally, using a stochastic simulation, we 

evaluate the sensitivity of the final outcome of the predictive model to uncertainty. We provide 

guidance for categorizing question-level uncertainty and summarizing it for the risk elements 

below, and guidance for conducting the stochastic simulation in Process 6. 

Uncertainty ratings in the PPQ WRA. For every answer in the PPQ WRA, the analyst must rate 

the level of uncertainty associated with their answer as either negligible (= negl), low, moderate 

(= mod), high, or maximum (= max). These qualitative descriptors describe your confidence in 

the answers based on the quality and quantity of the evidence. When evaluating uncertainty, 

analysts should consider the following factors: evidence quality, source of the evidence, 

availability of quantitative information, complexity of the trait being evaluated, independence of 

sources, expert opinion, use of congeneric information, conflicting information, and whether 

inferences and assumptions were necessary. As a general rule, the availability of quantitative 

data, multiple independent sources, and peer-reviewed sources should lower uncertainty (Fig. 8). 

In contrast, uncertainty should increase if evidence is unavailable, or if answers are based on 

conflicting evidence, congeneric information, “low quality” or non-peer-reviewed sources, or 

analyst interpretations. Conjecture, speculation, and untested hypotheses are not considered 

evidence, but where appropriate they may be used to support particular answers, albeit with 

much higher levels of uncertainty. 

Some additional points about the uncertainty associated with the use of congeneric information 

are worth making. First, using evidence about congeners with similar biology is inherently more 

uncertain. Consequently, when an answer is based solely on congeneric information, the analyst 

should increase the uncertainty by at least one level. In contrast, if there is some evidence about 
TP E-300, Ver. 2.3 February 11, 2019 33

the taxon being assessed and it is weak, information from several congeners might be used to 

reduce uncertainty. This can be done for both the Establishment/Spread and Impact Potential risk 

elements. Finally, increasing uncertainty may be appropriate if one uses descriptions that apply 

to the entire genus (e.g., the genus Vaccinium produces fleshy fruits that are dispersed by birds). 

As a general rule, if a single piece of direct evidence for the taxon from a reputable or peerreviewed source supports a particular answer, start with low uncertainty. Then, consider how 

the other factors summarized in Figure 8 affect the level of uncertainty. When the answer is 

clear-cut, the uncertainty level is likely to be low or negligible. Still, only use negligible 

uncertainty when the evidence is very strong and leaves very little doubt. When the answer is not 

so clear-cut, usually because of conflicting, indirect, or congener-based evidence, the uncertainty 

level is more likely to be moderate or high (see below). 

If you select an answer of “no” based on a lack of evidence, start with moderate uncertainty. 

Then, consider the other factors described in section 3-2 under “Answering ‘no’ based on a lack 

of evidence” (Fig. 8) to determine the uncertainty level, as follows: 

 If the taxon is very well known because it either has been extensively studied or is widely 

cultivated, consider reducing uncertainty. 

 If limited information exists on the taxon, consider increasing uncertainty. 

 If the trait or biology being evaluated is simple or straightforward, consider reducing 

uncertainty. 

 If the biology is more complex, not readily observable, or not likely to be reported, 

consider increasing uncertainty. 

 If reasonable speculation suggests that the answer may or could be yes, but you found no 

direct or indirect evidence to support a yes response, increase uncertainty. 

 If a lot of uncertainty exists, and you think yes and no seem equally likely, then answer 

unknown with maximum uncertainty. 

We think that it is generally inappropriate to assign negligible uncertainty when an answer is 

based on a lack of evidence. 

Assuming analysts follow the general rules for assigning uncertainty, you do not need to explain 

in every case why you chose a particular level of uncertainty. However, if the situation for any 

given question is complex, unclear, or involves conflicting information, you should explain at the 

end of the evidence why you chose that particular level of uncertainty. This explanation not only 

ensures transparency in risk assessment, but also identifies opportunities for reducing uncertainty 

in the future. 
TP E-300, Ver. 2.3 February 11, 2019 34

Taxon not new 

Lack of 

Evidence 

Taxon is new or recently bred 

Taxon is widely studied or cultivated Taxon is not well known 

Taxon introduced beyond native range Taxon still in native range 

Simple trait, readily observable Complex trait or biology 

Direct evidence 

Direct or Indirect 

Evidence

Conflicting information 

Quantitative data Congeneric information 

Multiple independent sources “Low-quality” sources 

Expert information Indirect evidence & interpretations 

Simple biological traits Complex biological traits 

Figure 8. The general impact of factors on the level of uncertainty associated with an answer. 

3.4. Entering the level of uncertainty 

Just as when entering answers into cells, we recommend typing the uncertainty rating (negl, low, 

mod, high, or max) into the cell in the “Uncertainty” column of the “Screening Tool,” but you 

can also select it from the dropdown box within the cell. To ensure that the workbook formulas 

function properly, enter the uncertainties in lowercase text exactly as indicated and do not use 

periods with the abbreviations. The workbook automatically tracks how many uncertainty ratings 

have not been entered (cell C136 in Fig. 7). If an uncertainty rating has been entered but was not 

written correctly (e.g., “negl.” instead of “negl” or with spaces before/after the rating), then it 

will count as an incomplete entry. 

Besides their qualitative meaning for analysts, the uncertainty ratings also describe the likelihood 

of an alternative answer being correct. For the purpose of calculating an index of uncertainty for 

each risk element (see next section), we assigned each uncertainty rating a probability value that 

represents that likelihood (Table 4). If the uncertainty rating is maximum (only possible when a 

question is answered as unknown), then all possible answers for the question are equally likely. 

For the purpose of conducting a stochastic simulation of the impacts of uncertainty (see Process 

6), we also assigned each uncertainty rating a probability distribution around the static 

probabilities (Table 4). We did this primarily because the static probability values were 

arbitrarily chosen. Overall, the static probabilities and their associated ranges represent 

probabilities from near zero (negligible uncertainty) to a 50 percent chance (high uncertainty). A 

detailed explanation and validation of our uncertainty model is forthcoming (Caton et al., in 

review). 

Decreasin

g uncertaint

y

Increasin

g uncertaint

y
TP E-300, Ver. 2.3 February 11, 2019 35

Table 4. Uncertainty ratings used to describe question-level uncertainty in the PPQ WRA. The 

probability associated with a rating represents our belief that some other answer is correct. 

 Probability that an alternative answer is correct 

Uncertainty Level Probability Range 

Negligible (=negl) 0.001 0 to 0.005 

Low 0.01 0.005 to 0.05 

Mod (=mod) 0.1 0.05 to 0.175 

High 0.25 0.175 to 0.5 

Max (=max) All answer choices are equally likely 

3.5. The index of uncertainty

We estimate the overall level of uncertainty associated with each risk element— 

Establishment/Spread, Impact, and Entry Potential—with a calculated “index of uncertainty.” 

The index accounts for both the uncertainty associated with each question and the weight of each 

question in the risk element score. Questions that are weighted more heavily in the WRA model 

(i.e., have potentially greater score values) contribute more to the index. The index of uncertainty 

is unit-less and ranges between 0 and 1. A value of 1 indicates maximum uncertainty and reflects 

a situation in which all of the questions have been answered as unknown. A value of 0 represents 

complete certainty. The index for each risk element is calculated automatically in the workbook 

and is presented in the “Uncertainty” column of “Risk Score Summary” section (cells D127-

D134) of the “Screening Tool” worksheet (Fig. 7). 

In the risk element summaries of the Word template, we report the index of uncertainty for the 

Establishment/Spread, Impact, and Entry Potential risk elements. However, because readers may 

not understand what this index means, we conclude each written summary by describing the 

level of uncertainty in the following manner: “We had a [very low, low, average, high, or very 

high] level of uncertainty for this risk element.” We defined the value ranges for the different 

categories based on data for the 204 species that were used to develop and validate the PPQ 

WRA model. The uncertainty score distributions for the Establishment/Spread and Impact 

Potential risk elements were very similar and had an average index of about 0.17 (Caton et al., in 

review). The thresholds were chosen based on the 20th, 40th, 60th, and 80th percentiles of these 

distributions (Table 5). Until the Entry Potential risk element is validated, we are assuming that 

the score distribution for its index of uncertainty will be similar to that of the other two risk 

elements. 
TP E-300, Ver. 2.3 February 11, 2019 36

Table 5. Thresholds for different qualitative levels of uncertainty for the Establishment/Spread 

and Impact Potential risk elements. 

Index of Uncertainty 

Score Range 

Qualitative 

Description of the 

Level of Uncertainty 

Index < 0.07 Very low 

0.07 ≥ Index < 0.13 Low 

0.13 ≥ Index < 0.20 Average 

0.20 ≥ Index < 0.26 High 

Index ≥ 0.26 Very High 

3-6. Tracking the process in the spreadsheet 

As noted above, the “Screening Tool” worksheet tracks how many questions and uncertainties 

remain to be answered (see cells C135 and C136; Fig. 7). These cells will remain highlighted red 

until all the questions and uncertainties have been entered, including the 10 alternate answers to 

the five multiple-choice questions. 

Process 4: Assessing geographic potential 

Predicting the geographic potential (i.e., areas with suitable habitat for establishment and spread) 

of a plant taxon is an important component of any WRA, as climatological factors affect the 

ability of a taxon to establish in any given area. We assess geographic potential in the PPQ WRA 

using Proto3, an additive, raster-based2 prototype model, in ArcGIS. Proto3 uses variables from 

three global datasets (climate layers) representing plant hardiness zones, mean annual 

precipitation (in 10-inch bands), and Köppen-Geiger climate classes to predict where a given 

taxon may find suitable habitat in the United States and its territories. Analysts use a variety of 

sources to gather georeferenced presence data (data with latitude/longitude coordinates) and 

plant taxon occurrences that do not have specific coordinates (reported occurrences). Using the 

data and the spatial layers, Proto3 creates a map of the United States where the layers overlap. 

Figure 9 provides an overview of the Proto3 model. In spatial analysis terms, the response 

variable (derived from georeferenced and reported occurrence data for the taxon) informs the 

creation of the predictor variables (using the three climatic spatial layers), which are run through 

the model algorithm in ArcGIS to produce the predicted map. In other words, we provide data, 

create new layers, combine them, and generate a predicted map, as further described in the 

sections below. 

 

2

 A raster provides data in the form of cells or pixels. In the case of our geographic potential assessment, it treats the 

map area as a grid and associates data with each cell. 
TP E-300, Ver. 2.3 February 11, 2019 37

Figure 9. Overview of the PPQ Proto3 model for assessing the geographic potential of a plant 

taxon. 

Our plant hardiness zone and annual precipitation datasets were provided through the North 

Carolina State University/Animal and Plant Health Inspection Service Plant Pest Forecasting 

System. These datasets were generated using Climate Forecast System Reanalysis (CFSR) base 

data (Saha et al., 2010), following the procedures outlined in Magarey et al. (2008). We use two 

data sets for each variable, one representing 30-year averages and the other representing 10-year 

averages. We use the 30-year datasets to gather the data for the Proto3 model to answer the 

questions in the Geographic Potential section of the Excel workbook and the 10-year datasets to 

produce the predictive maps. Many of the global data points representing plant distribution were 

recorded over several decades, and the 30-year datasets capture a broad range of climatic 

variance during those decades. The 10-year datasets represent more recent estimations of 

minimum temperatures and mean precipitation. Using the 10-year maps to produce our 

predictive maps allows us to provide a current estimation of potential distribution and 

incorporate climate change into our model. The Köppen-Geiger climate class dataset is an 

adaptation of a raster made available online by Peel et al. (2007). The original dataset contained 

32 climate classes that we reclassified to represent 12 specific but more broadly defined classes 

as described by Espenshade et al. (1995) (e.g., tropical rainforest, desert, tundra). 

The guidance in the following sections is an overview of the process for evaluating geographic 

potential. We provide instructions for downloading the data and creating the Proto3 maps in a 

detailed instruction manual, which is available upon request. Beyond this general guidance, if 

priorities allow we can also train analysts on how to gather the georeferenced and reported 

occurrence data and run the model in ArcGIS. 

4-1. Gathering and preparing the data for input into ArcGIS 

Gather data about the distribution of the taxon from a variety of sources. At the start of the WRA 

process, we recommend that you download all occurrences from the Global Biodiversity 
TP E-300, Ver. 2.3 February 11, 2019 38

Information Facility (GBIF) (see section 1-4). This database provides two types of records: 

georeferenced data (with latitude and longitude coordinates) and reported occurrence data (local 

or regional occurrences without specific coordinates). Copy all of these data into the GBIF 

worksheet of the Excel workbook, and take note of where the taxon is reported to occur 

throughout the world, including countries, provinces, states, cities, etc. These data supplement 

georeferenced points and improve the assessment of the taxon’s known global distribution, 

particularly if that distribution was not captured in the GBIF database. The more precise the 

location data, the lesser the uncertainty. 

After reviewing the majority of the literature for the taxon, begin assessing geographic potential 

by organizing and cleaning the GBIF data you downloaded to the workbook. Remove duplicates 

and reports of plants under cultivation and ensure that geographic coordinates match countries. 

As mentioned above, we can provide detailed work instructions for this in a separate instruction 

manual (Assessing Geographic Potential Instruction Manual) upon request. 

4-2. Adding georeferenced and reported occurrence data to ArcMap 

To conduct the geopotential evaluation, you need access to a computer with ArcGIS software 

and a map document that contains all datasets needed for the analysis. We provide each analyst 

with their own folder and map document in ArcMap, and the analyst creates new datasets for 

each taxon assessed. 

Import the georeferenced data (latitude/longitude coordinates—called “xy” data in ArcGIS) 

contained in the GBIF worksheet into an ArcMap document. The coordinates show up on the 

map as points scattered across the globe. Polygons outlining country, province, state, or county 

boundaries can be created on the map to represent the occurrence data. These layers, which 

represent the global distribution of the taxon, are then used to answer the questions related to the 

taxon’s presence in a given climatic region. 

4-3. Assessing the global distribution of a taxon 

Three climatic variables are used to assess the geographic potential of a plant taxon: plant 

hardiness zones, Köppen-Geiger climate classes, and mean annual precipitation3

. In this step, 

you will overlay the data layers describing the taxon’s global distribution over each of the three 

climate maps to determine where (in which hardiness zones, climate classes, and precipitation 

bands) the taxon can occur. You will then answer yes or no to each Geographic Potential 

question in the “Screening Tool” worksheet and describe your uncertainty as negligible, low 

moderate, or high. Provide evidence for your answers in the “Notes (and references)” column. 

For example, if you have evidence that your taxon occurs in a given Plant Hardiness Zone in four 

countries, list those countries in the comment field. Note that you cannot answer “unknown” in 

the geographic potential analysis; make the best determination for a yes or no answer based on 

the available evidence. Question-specific guidance is available in Appendix G. 

When evaluating the uncertainty associated with each question in this element, consider the type 

of data available (georeferenced or reported occurrences), the quantity of points for any given 

climate layer (i.e., zone, band, class), if the data are available from multiple countries, and if the 

 

3

 There are 13 Plant Hardiness Zones, ranging from Zone 1 in the coldest climates to Zone 13 in the tropics. We use 

12 climate classes in the Köppen-Geiger dataset. Mean annual precipitation is represented by 11 ten-inch bands. 
TP E-300, Ver. 2.3 February 11, 2019 39

data are consistent with the overall biology of the taxon. For example, if a climate class has five 

points in each of three different countries, you might reasonably answer yes with negligible 

uncertainty. If only a few points exist, but the taxon being present in that zone or class seems 

reasonable, based on the biological traits of the taxon and its native distribution, answer yes with 

low uncertainty. 

Using the reported occurrence data is more art than science and requires the analyst’s expertise. 

For example, if evidence indicates that the taxon is found in China or Russia, answering yes for 

every climate class or Zone in those large countries would likely overestimate the true 

distribution. Consider other evidence and biological traits encountered while answering 

questions for the predictive model. For example, are there extreme temperatures under which the 

taxon cannot survive? This is good evidence to support an answer of no. A specific search for the 

taxon in a given province may help to narrow down a large area by providing county-level 

occurrence data. Although time-consuming, using the reported occurrence data in addition to 

georeferenced points in the assessment may often give a more accurate representation of the area 

at risk. The use of the reported occurrences (i.e., data without georeferenced points) distinguishes 

our Proto3 method from other spatial analyses that depend strictly on georeferenced data. 

Occasionally, the reported occurrence data will not add any new information, in which case only 

the georeferenced data will be used in the Proto3 analysis. 

4-4. Conducting Proto3 analysis 

Once all questions are answered, follow the Weed Team’s Assessing Geographic Potential 

Instruction Manual to complete the Proto3 analysis and create the map specific to the plant 

taxon. The general process is as follows: 

1. Create global maps for each of the three climate variables (Plant Hardiness Zones, 

Köppen-Geiger climate classes, mean annual precipitation) representing areas 

where the taxon could establish. 

2. Create a new map in which the three variables overlap. 

3. Create a U.S.-centric map and calculate the predicted area (in square miles or 

square kilometers). 

4. Record the area in the Maps worksheet in the Excel workbook. The spreadsheet 

automatically estimates the percentage of the U.S. area that is suitable for 

establishment. 

5. Create an image (in appropriate software, e.g., PowerPoint) of your predicted 

areas for use in the WRA Word document. 

Final notes 

The area in red (Fig. 9) represents an estimate of areas in which the climate may be conducive 

for the establishment of a given taxon. Other variables may further limit where a taxon can 

establish. Alternately, the taxon may be able to establish in microhabitats not represented by the 

predictive map. If a taxon were to establish in the predicted range, it is unlikely that it would 

become invasive throughout the entire range as climate and other variables may be more limiting 

towards the periphery of its range. 
TP E-300, Ver. 2.3 February 11, 2019 40

Proto3 is a simple climate-matching analysis; other, more detailed tools are available. We 

validated the Proto3 model using species naturalized in the United States and, using the same 

data, compared the accuracy of the Proto3 model to three other species distribution models that 

are more complex [MaxEnt and CLIMEX (Match Climate), and a process model known as the 

Thornley Transport Resistance Model]. The results from this initial study showed that Proto3 

performed as well than these modeling systems and can be used for assessing geographic 

potential (Magarey et al., in review). 

Process 5: Assessing entry potential 

The entry potential section of the PPQ weed risk assessment has not yet been validated; this risk 

element may change considerably once that happens. 

The entry potential section consists of 14 questions that assess the likelihood that the plant taxon 

will enter the United States intentionally for cultivation, accidentally as a contaminant, or 

through natural dispersal. Taxa that are cultivated elsewhere or are positively valued by society 

are more likely to enter than other, less valued, taxa. The risk score for this element ranges from 

0 to 1, with higher scores indicating a higher likelihood of entry. 

If a taxon is already present in the United States, or if a permit for its entry has been requested, 

the analyst only answers the first (Ent-1) and potentially the second (Ent-2) question for this risk 

element. Assessing the entire risk element is not necessary in such cases, because after answering 

yes to either Ent-1 or Ent-2 the Excel workbook automatically sets all other scores as not 

applicable and thereby prevents the analyst from answering any more questions. 

If the taxon is present in the United States but has a limited naturalized distribution or is not 

widely cultivated, evaluating entry potential may be prudent. In such cases, you can answer no to 

Ent-1 and Ent-2 and explain in the “Notes (and references)” column why you are fully evaluating 

the entry potential. Alternatively, you can answer yes to Ent-1 or Ent-2 and not answer the rest of 

the questions while still providing evidence for all the questions in the “Notes (and references)” 

column. The evidence will be available to risk managers if they deem it helpful for their 

decision-making process. 

Sources and information used elsewhere in the WRA will be useful to answer the questions in 

the Entry Potential risk element. 

 To answer Ent-1, use the references listed under Process 1-4 to verify the U.S. 

distribution of the plant taxon. 

 Review your evidence from ES-1 (Establishment and spread status outside native range), 

ES-2 (Is the species highly domesticated?), ES-15 (unintentional spread by human 

activity), ES-16 (contaminant of agricultural products), ES-17 (dispersal vectors), and 

Imp-P3 (trade impacts). 

For guidance and information related to each question, refer to the question-specific guidance 

(Appendix H). 
TP E-300, Ver. 2.3 February 11, 2019 41

Process 6: Uncertainty analysis 

In Process 6, we conduct a stochastic simulation to evaluate the sensitivity of the risk scores to 

uncertainty in the assessment—specifically, uncertainty in the answers to the questions in the 

Establishment/Spread and Impact Potential risk elements. A stochastic simulation is a 

computational technique in which repeated random samples are taken from a dataset to evaluate 

how likely an outcome or score is. Here, we refer to the stochastic simulation as the uncertainty 

analysis. In the case of the PPQ WRA, the uncertainty analysis helps us evaluate what would 

happen to the risk scores if we chose a slightly different set of answers. Hence, the analysis 

allows us to evaluate what other risk scores are likely given the uncertainty in the WRA. The 

greater the uncertainty in the WRA, the wider the range of possible risk scores around the 

original analyst-derived score. For a full explanation of how we developed and tested the 

underlying uncertainty model see Caton et al. (in review). 

In the uncertainty analysis, the software stochastically determines if each answer has changed 

based on its uncertainty rating. The greater the question uncertainty, the more likely the chosen 

answer will change to an alternate answer. After determining which answers are changing, the 

simulation chooses new answers, based on their likelihoods. For yes/no questions, the new 

answer is simply the opposite, while for multiple-choice questions the two alternate answers are 

equally likely. If a question was answered as unknown, all answer choices for that question are 

equally likely. After all answers have been determined, the model calculates the new risk scores, 

determines the outcome of the assessment, and runs the secondary screening analysis if 

necessary. It repeats this process 5,000 times, and we graph the resulting simulated risk scores 

and ratings outcomes, as well as the analyst-derived risk score. Appendix I describes the 

uncertainty analysis in greater detail. 

Before beginning the uncertainty analysis, ensure that all questions have been answered and all 

uncertainty ratings correctly filled by referring to cells C135 and C136 (Fig. 7) in the “Screening 

Tool” worksheet. 

PPQ uses the software program @Risk to conduct the uncertainty analysis. This program is 

installed as an add-in for Microsoft Excel and must be purchased separately from the software 

company Palisade. The specific stochastic model that PPQ uses to conduct the uncertainty 

analysis is already built into the workbook in the “Uncertainty” worksheet. If the correct 

software is already installed, the analyst only needs to select “Start simulation” from the menu 

under the @Risk tab in the workbook to run the analysis. Appendix J provides specific work 

instructions for conducting the analysis, which takes only five minutes. If you are running the 

uncertainty analysis for the first time on a given computer or previously ran @Risk using 

different settings, you may need to reset a few simulation settings as described in Appendix I. 

Because several of the worksheets in the workbook are password protected to prevent accidental 

changes to the formulas, you will need to enter the password “test” when prompted by @Risk. 

Entering this password within an @Risk simulation does not unprotect the worksheets; rather, it 

gives the software permission to access them. 

Once the @Risk simulation has completed, incorporate the relevant data from the output into the 

“Risk Score Plot” worksheet following the guidance in Appendix J. We built this worksheet to 

automatically graph the results of the simulation. We developed a standard method for displaying 
TP E-300, Ver. 2.3 February 11, 2019 42

the results in the Establishment/Spread and Impact Potential risk space (Fig. 10). The results 

from the 5,000 iterations are displayed as 50th, 95th, and 99th percentile boxes, with the remaining 

1 percent of the simulated risk scores plotted as outliers. The median values for the 

Establishment/Spread and Impact Potential risk scores are shown as a blue cross within these 

boxes. The original risk score that was determined by the analyst is plotted as a solid black 

square, along with the two decision thresholds that separate the risk ratings. For transparency, we 

also report the percentage of the 5,000 simulated risk scores that resulted in each of five possible 

outcomes, incorporating the results from the secondary screening: 1) Low Risk, 2) Evaluate 

Further  Low Risk, 3) Evaluate Further  Evaluate Further, 4) Evaluate Further  High Risk, 

and 5) High Risk. 

Figure 10. Model simulation results (N=5,000) for uncertainty around the risk score for Hakea 

europaeum.

At a quick glance, the figure summarizing the results of the uncertainty analysis provides several 

key pieces of information important for risk analysts and decision-makers to consider. Analysts 

may use this information to determine when additional information is needed to reduce 

uncertainty. Decision-makers may use this information for making regulatory decisions or 

determining the strength of mitigation measures: 

 The overall size of the boxes. Box sizes depend upon the amount of uncertainty in the 

assessment. They correlate well with the index of uncertainty, so that greater uncertainty 

gives larger boxes, and vice versa. Greater uncertainty for one risk element or the other 

will produce less square, more elongated rectangular boxes (Fig. 11). 

 The location of the analyst-derived risk score relative to the center of the distribution of 

simulated risk scores. Ideally, the WRA has low uncertainty and the analyst-derived risk 

score will be located near the intersection of the medians (blue lines). However, this 

1

1.5

2

2.5

3

3.5

4

4.5

5

‐20 ‐15 ‐10 ‐5 0 5 10 15 20 25

Impact Potential

Establishment / Spread Potential

Low Risk High Risk Evaluate 

Further

High Risk

EF→High 

EF→EF

EF→Low

Low Risk

15.5%

84.4%

0.2%

0%

0%
TP E-300, Ver. 2.3 February 11, 2019 43

seems to rarely be the case. Often, the risk score is located off-center, usually left and 

below, and sometimes outside some or all of the percentile boxes (Fig. 11). This is 

expected and merely indicates that if you take uncertainty into account, picking a slightly 

different set of answers, the new risk scores tend to increase. If additional information 

changed one or more answers, the new risk score would likely move toward the center of 

the distribution. However, additional information may only reduce the uncertainty in the 

WRA, giving smaller and more centered boxes around the original risk score (Caton et 

al., in review). 

 The distribution of the simulated risk scores with respect to the decision thresholds. 

Regardless of the amount of uncertainty in a WRA, if the original risk score and all of the 

simulated risk scores are located within either the Low Risk or High Risk regions of the 

graph, then the conclusion from the WRA is well supported. If the results do not meet 

these criteria, then consider the next bullet. 

 The dominant risk rating outcome for the simulated risk scores. The inset in the 

uncertainty figure displays the percentage of the 5,000 iterations among the different 

possible outcomes (i.e., ratings). If distributions cross one or both decision thresholds or 

are located entirely within the Evaluate Further risk region, these results are important to 

examine. If the most frequent outcome coincides with the original risk rating, then the 

original rating is statistically robust. If they differ, then the results are somewhat 

conflicted. During validation of the uncertainty model (Caton et al., in review), 13 

percent of the 204 species analyzed had dominant outcomes in the uncertainty analysis 

that did not support the original risk rating. Sometimes it may not be possible to resolve 

the conflict; in such cases, noting the discrepancy for decision-makers to consider is 

prudent and acceptable. 
TP E-300, Ver. 2.3 February 11, 2019 44

Figure 11. Representative uncertainty analysis charts for different levels of uncertainty in 

Establishment/Spread Potential (x-axis) and Impact Potential (y-axis). 

Process 7: Finalizing and assembling the WRA 

At this point, you should have answered all of the questions in the predictive WRA model, 

evaluated the sensitivity of the final risk rating to uncertainty, determined where in the United 

States the taxon is likely to establish, and if applicable, evaluated its likelihood for entry. In 

Process 7, you will review the final outcome of the assessment and formally finalize the analysis 

using the Word template. 

7-1. Reviewing the final outcome of the assessment in the Excel workbook 

Before finalizing the WRA Word template, review the results in the workbook as outlined in the 

following bullets. 

 Review the final results of the WRA model as shown by cells C139 to C143 on the 

“Screening Tool” worksheet. All of these results are automatically calculated by cell 

formulas. The three probabilities of invasiveness (cells C139 to C141) are the direct 

results from the logistic regression model that predicts weed risk (see Appendix B). Once 

all of the questions have been answered, the result is automatically calculated in the 

worksheet (C142, Fig. 7), including the result of the secondary screening tool (C143, Fig. 

7) if the primary result was Evaluate Further. As part of the secondary screening process 
TP E-300, Ver. 2.3 February 11, 2019 45

(see Appendix D), we calculate the “Secondary Screening Score,” which is displayed in 

cell C137. 

 Consider the results from the previous bullet as well as the two figures on the “Risk Score 

Plot” worksheet. Are these results consistent with the accumulated evidence? In other 

words, do the results make sense based on the overall weight and diversity of evidence? 

If not, then review the evidence, answers, and uncertainties to determine if some of the 

questions were answered too liberally or conservatively. 

 Consider the final risk rating from the WRA model (i.e., cell C142 or C143, depending 

on if the secondary screening was necessary, Fig. 7) and the dominant risk outcome from 

the uncertainty analysis (e.g., see Fig. 4). If they differ, consider reviewing the literature a 

little more extensively to see if you can reduce uncertainty for some key questions. 

Contacting experts who have experience with the taxon under field conditions may be 

helpful. Additional analysis may not be necessary if you think the final risk rating and the 

dominant outcome from the uncertainty analysis are likely to lead to a particular decision 

by policy managers. Consulting with the policy managers—even before finalizing the 

WRA—may also be prudent, to determine their comfort level and understanding of the 

information presented. 

 Review the map showing the geographic potential of the taxon in the United States (e.g., 

see Fig. 3). Does the map seem reasonable with respect to the general biology and 

adaptations of the taxon? If not, review the evidence and the answers for the geographic 

potential risk element and revise the map accordingly. 

 Evaluate the final risk rating of the WRA. Was it expected, or do you think the risk was 

over- or underestimated? Is the result consistent with the weight of the evidence? In other 

words, do the risk scores and the final model result make sense? If necessary, review the 

assessment and determine if you were too conservative or too liberal with your answers. 

Consider whether high uncertainty might have affected the results. 

 If you change any of the answers or uncertainties, you will need to re-run the uncertainty 

analysis. 

7-2. Preparing and finalizing the WRA Word document

The results of the work conducted in the workbook, as well as background information on the 

taxon, are presented and summarized in a WRA document. It includes the cover page, 

introduction, taxon information, summary of the analysis, analytical results, discussion section, 

literature cited, and appendix of the data from the workbook. If you have not already done so, 

save a copy of the template that is used to create the document to your computer and rename it 

with the scientific name of the plant taxon being assessed (i.e., Genus species.docx). The 

template provides some standard text, such as section headings and the introduction text, that is 

applicable to all WRAs and therefore does not need to be modified. The green highlighted areas 

in the template indicate places where the analyst needs to fill in information for their particular 

WRA. These highlighted areas provide instructions as to what needs to be entered in that section 

of the document. Complete each section of the template by following the instructions in the 

template itself as well as the instructions provided below. As you enter information in the 

template, delete the instructional text and remove any green highlights. 

Taxon background information. When making regulatory decisions, decision-makers consider 

basic information about the taxon that is presented on the cover page and in the “Species 
TP E-300, Ver. 2.3 February 11, 2019 46

Information” section of the Word template. At this point in the WRA process, you will have 

probably already completed recording some of this information (i.e., taxonomy, common names, 

and the initiating event in the “Species Information” section), as instructed in Process 1 

(Gathering Background Information). If you did not already fill out this information in the Word 

template, refer back to Process 1, sections 1-1 and 1-3 for guidance on doing this. Otherwise, 

finalize the “Species Information” section and prepare the cover page by following the guidance 

below as well as that contained in the template. Ensure that all information is appropriately cited. 

 On the cover page, ensure that the current scientific name and most representative 

English common name are included. Also, include one or more images that show the life 

form, habit, and/or habitat of the taxon. These images should complement the taxon’s 

botanical description (see below). If available, include an image of a population of the 

taxon that represents the kind of impacts it may be having. Caption image(s) and credit 

them to the appropriate source. Many websites will provide guidance on how exactly to 

acknowledge the source. Only use images that are under a common license agreement or 

that you have obtained permission to use. 

 In “Species Information,” incorporate any additional information on taxonomy, including 

taxon name, family, synonyms, and common names that you may have come across as 

you gathered evidence for the rest of the WRA process. 

 In “Botanical description,” in one to three sentences, provide a concise botanical 

description of the taxon so that someone not familiar with it will have a general 

understanding of what kind of plant was assessed [e.g., size, life form (herb, vine, tree, 

grass), life cycle (annual, perennial), preferred habitats] (see Process 1-2 for further 

guidance). Cite a couple of references, preferably floras, where the reader can obtain a 

full botanical description. Consider the following botanical description as an example: 

“Heliotropium europaeum is an erect or semi-prostrate branched annual growing 10 to 50

cm tall and produces a well-developed taproot (Holm et al., 1997; Parsons and 

Cuthbertson, 2001). It is relatively hairy, produces flowers in indeterminate scorpioide 

cymes, and has a somewhat offensive odor when crushed (Parsons and Cuthbertson, 

2001). For a full description see Zhengyi et al. (2014).” 

 In “Foreign distribution and status,” briefly describe the taxon’s foreign distribution 

distinguishing between its native origin, introduced range, and naturalized/invasive range. 

If the number of countries of distribution is not too large, list all of them. When 

appropriate, instead of listing individual countries, you can describe the taxon’s range in 

terms of regions (e.g., “this species is native to sub-Saharan Africa”). State whether the 

taxon has been moved beyond its native range. The information in this section will be 

somewhat redundant with the evidence used to answer question ES-1. Ensure that they 

are consistent with each other. State whether the taxon is cultivated beyond the United 

States and cite a few references. Also, briefly describe whether it is regulated or managed 

elsewhere. 

 In “U.S. distribution and status,” summarize all of the information relating to U.S. 

distribution, cultivation, and management (see Process 2-4 for detailed guidance on 

gathering this information) in a single paragraph. This section is meant to be a summary 

for decision-makers and risk managers. If the plant taxon is not present in the United 

States, state that it is not known to be present and cite the major sources of information 

used to determine this. If the taxon is present in the United States, state when, how, and 

why it was introduced, if this information is readily available. If cultivated, describe the 
TP E-300, Ver. 2.3 February 11, 2019 47

extent to which it is cultivated. Describe if it is naturalized, and if any states or other 

agencies, groups, or individuals are managing it. List the states where it is naturalized. If 

it is naturalized in many states, you can simply describe the region (e.g., the southeast) 

along with the number of states in that region where it occurs. If appropriate, describe 

any uncertainty relating to the status of the taxon. State whether additional research or 

evaluation is necessary to address any significant uncertainty. Additional details on status 

in the United States can be provided elsewhere in the WRA. 

 In “WRA area,” identify the area in relation to which the WRA is conducted [definition 

modified from that for “PRA area” (IPPC, 2013a)]. Because the predictive model was 

developed for the entire United States and because the WRA is geographically neutral (as 

explained in Section 2), the WRA area is always the entire United States, even if it is 

initiated for a particular state or region. If the WRA was conducted for another region of 

the world outside of the United States, identify that region here4

. 

Risk element summaries 

In the “Establishment/Spread Potential” and “Impact Potential” sections, provide a small 

summary highlighting the traits that contributed the most to the taxon’s risk score. Add other 

information related to the risk element that you think should be considered by decision-makers 

and risk managers but was not covered under any of the questions in the risk element. 

Additionally, include a brief statement or discussion about the amount of uncertainty for each 

risk element. For example, “We had a very low amount of uncertainty for this risk element,” or 

“We had a high amount of uncertainty for this risk element due to....” Refer to Table 5 (Process 

3) to determine the appropriate qualitative descriptions to use for your taxon’s uncertainty scores. 

Finally, add the risk scores and the uncertainty index scores where indicated in the template. 

These scores can be found in the Risk Score Summary section of the “Screening Tool” 

worksheet. 

In “Geographic Potential,” provide the estimated percentage of the United States suitable for 

establishment of the taxon and list the climate zones and classes that make up that predicted 

distribution. As needed, discuss any major sources of uncertainty in the analysis. For instance, in 

cases where there was a lot of uncertainty for a given hardiness zone, climate class, or 

precipitation band and a different answer would have affected the area of the predicted 

distribution, discuss the source of the uncertainty and how you addressed it. Providing a narrative 

explanation of such uncertainties is important for this risk element, because—unlike for the other 

risk elements—we cannot calculate an index of uncertainty for it. To help explain how other 

environmental variables may limit the predicted distribution for the taxon, describe its preferred 

habitats. For example, “This taxon is restricted to lake shores and other riparian areas,” or “Its 

habitats include hot, dry, low-lying areas, and savanna woodlands.” 

 

4

 Because the predictive model of the PPQ WRA is not restricted to any given geographic region, it may be used 

anywhere in the world, provided that a climate-matching analysis confirms the WRA area is suitable for 

establishment by the plant taxon under assessment. Many of the questions in the PPQ WRA were derived from the 

Australian WRA, which has proven to be reasonably accurate for a number of regions around the world (Gordon et 

al., 2008). Thus, we expect that the PPQ WRA will perform similarly. However, some of the individual questions in 

our process may not be as predictive of invasive status in the foreign country as they are for the United States due to 

unique ecological processes that influence species invasiveness in other biogeographic regions. 
TP E-300, Ver. 2.3 February 11, 2019 48

In “Entry Potential,” if a full assessment of this risk element was not necessary (that is, you 

answered yes for either Ent-1 or Ent-2), enter the following standard text: “We did not assess the 

entry potential of Taxon X because it is already present in the United States (citations).” Also, 

delete the line containing the placeholders for the entry potential risk score and uncertainty 

index. If the entry potential risk element was fully assessed, then follow the same guidance as 

that provided above for the “Establishment/Spread Potential” and “Impact Potential” sections. 

For Figure 1 (Predicted distribution of [Taxon] in the United States): Go to the “Maps” 

worksheet, copy the image showing the predicted U.S. distribution of the taxon, and then paste 

this image into the box for Figure 1. [This map image is produced in ArcGIS based on the 

detailed instructions provided in the separate geographic potential instruction manual (as 

mentioned in Process 4).] Resize the image as needed to fit the box. 

Results

In the “Results” section, enter the following information: 

 The model probabilities for the taxon being a major-invader, minor-invader, and noninvader. These probabilities can be found in the “Risk Score Summary” section of the 

“Screening Tool” worksheet (see cells C139-141). In the workbook, these probabilities 

are presented as decimal numbers. When adding them to the template, be sure to change 

these to percentages (e.g., 0.972  97.2%). 

 The Risk Result from the logistic regression model, which is either Low Risk, Evaluate 

Further, or High Risk. This result is indicated in the “Risk Score Summary” section of the 

“Screening Tool” worksheet (see cell C142). 

 If the result from the logistic regression model was Evaluate Further, under the result of 

the Secondary Screening enter Low Risk, Evaluate Further, or High Risk; otherwise, 

enter Not Applicable. The appropriate result is indicated in the “Risk Score Summary” 

section of the “Screening Tool” worksheet (see cell C143). 

 For Figure 2 (Taxon risk score): Go to the “Risk Score Plot” worksheet and copy the 

chart that contains the risk score for the taxon plus the scores from the 204 species used 

to develop and validate the WRA model (i.e., the top chart in the worksheet). In the 

template, select “Paste special”  “JPEG paste” from the menu to paste this chart into 

the designated space. If needed, add a border to the chart and/or resize the chart to best fit 

the space. 

 For Figure 3 (Model simulation results for uncertainty): Go to the “Risk Score Plot” 

worksheet and copy the chart that contains the taxon’s risk score and the results from the 

uncertainty analysis (i.e., the bottom chart in the worksheet). In the Word template, select 

“Paste special”  “JPEG paste” from the menu to paste this chart into the designated 

space. If needed, add a border to the chart and/or resize the chart to best fit the space. If 

possible, fit Figures 2 and 3 on the same page. 

Discussion

In “Discussion,” provide a summary and discussion of the overall result of the WRA, including 

the impact of uncertainty on the final conclusion. As part of this summary, be sure to refer to 

Figures 2 and 3 [e.g., “The result of the weed risk assessment for [Taxon] is High Risk (Fig. 2)” 

and “Overall, we are very confident in our determination of high risk based on the results of our 

uncertainty simulation (Fig. 3)”]. Additionally, describe any other information that may be useful 
TP E-300, Ver. 2.3 February 11, 2019 49

for risk managers and decision-makers to consider as they evaluate risk management options, 

including information that may suggest a different regulatory response than that indicated by the 

overall risk result. While we do not make any policy or management recommendations for 

regulating or deregulating species, we can make some suggestions on how to proceed with taxa 

that have a risk result of Evaluate Further. In general, this discussion section consists of one or 

two paragraphs and no more than three. 

Literature Cited

In “Literature Cited,” provide citations for all references cited in the WRA document, including 

those cited in Appendix A (see below). PPQ analysts should refer to instructions in the template 

for details on how to insert and format these references into the document. For all other analysts, 

we highly recommend that you use some sort of reference management software (e.g., EndNote) 

to manage your references and create the literature cited section. 

WRA Appendix

The last section of the Word template is Appendix A, which is a table of all the risk element 

questions and their associated answers, uncertainty scores, risk scores, and evidence/notes 

contained in the “Screening Tool” worksheet. The worksheet “TableForWord” formats this 

information so it can be copied into Appendix A from the spreadsheet. The only difference in 

content is that the text for most of the questions is abbreviated; otherwise, the answers, 

uncertainties, and notes are copied over exactly as they were entered. 

Follow these instructions for creating Appendix A (which are essentially the same as those 

provided in the template): Open the “TableForWord” worksheet in the workbook. Select the 

entire cell range that starts from the header row and ends with the last row under Entry Potential. 

Copy and paste that block of cells into the designated location in the template. You will most 

likely need to adjust the column widths to format it properly. When modifying column width, 

make sure you select the “+” symbol that appears when you hover over the left-top corner of the 

table to modify the entire table. Ensure the main header row (row1) repeats at the top of every 

page (right click the first row and select “Repeat as header” from Word’s table menu). Always 

scroll through to make sure everything is formatted properly. The font should be Times New 

Roman, size 10. 

Review the evidence for misspellings, grammatical errors, and clarity of writing. Because the 

evidence was not entered in a word-processing program, you will likely revise many of the 

entries in the appendix. Once you revise the text in the template, copy the cells corresponding to 

the evidence column, and paste them back over the cells in the workbook in the “Screening 

Tool” tab to ensure your edits are archived there as well. Pasting over the cells will likely change 

their formatting (i.e., font, cell wrapping), but this should not be a concern. Also, because the 

spacing between risk element sections in the workbook and the appendix is different, you will 

need to copy and paste the information for each risk element individually. Once you have copied 

over the information in the workbook, format the bibliography in the document using EndNote 

(or another reference management tool) so that references in the appendix are formatted and 

included in the bibliography. Once the reference section is finalized, copy it and paste it into the 

“Screening Tool” worksheet at cell H164. 
TP E-300, Ver. 2.3 February 11, 2019 50

Final steps 

As part of reading through and reviewing the WRA document, check that all the green 

highlighted instructions in the template have been deleted. Also, ensure that the green 

highlighted header on the cover page (“Form E-301...”) and Appendix B (Revision History) 

have been deleted. As noted above in Process 2, the PPQ WRA process is managed under a 

quality management system, and the template is one of the controlled documents in this system, 

so its version, effective date, and approver are recorded in the cover page header, and the 

revision history is recorded in Appendix B. Once the template is copied to create a WRA 

document for a particular taxon (i.e., a new WRA packet), this header and Appendix B should be 

deleted from the copy. 

7-3. Peer review of the final WRA product 

After you have completed the final WRA product (the workbook and the document), the product 

should undergo peer review. Peer review is vital to ensuring high quality-analyses by helping to 

identify potential mistakes in judgment and interpretation of the literature, to guard against bias, 

to maintain consistency between WRAs, and to improve clarity of the product. In PPQ, WRAs 

are produced by the Plant Epidemiology and Risk Analysis Laboratory (PERAL). All PERAL 

WRA products undergo scientific review by at least one weed risk analyst who has been trained 

in the PPQ WRA process, and are also reviewed by the PERAL editor and the PERAL assistant 

director (or their designee). 

Literature Cited 

7 CFR § 360. U.S. Code of Federal Regulations, Title 7, Part 360 (7 CFR §360 - Noxious Weed 

Regulations). United States Government. 

7 CFR § 361. U.S. Code of Federal Regulations, Title 7, Part 361 (7 CFR §361 - Importation of 

Seed and Screenings under the Federal Seed Act). United States Government. 

7 U.S.C. § 7701-7786. 2000. Plant Protection Act, Title 7 United States Code § 7701-7786. 

Acevedo-Rodríguez, P., and M. T. Strong. 2012. Catalogue of Seed Plants of the West Indies. 

Smithsonian Institution, Washington D.C. 1192 pp. 

Bailey, L. H., and E. Z. Bailey. 1976. Hortus Third: A Concise Dictionary of Plants Cultivated in 

the United States and Canada (revised and expanded by the Staff of the Liberty Hyde 

Bailey Hortorium). Macmillan, New York, U.S.A. 1290 pp. 

Booth, B. D., S. D. Murphy, and C. J. Swanton. 2003. Weed Ecology in Natural and Agricultural 

Systems. CABI, Wallingford, U.K. 303 pp. 

Brunel, S. (ed.). 2005. Proceedings of the International Workshop on Invasive Plants in 

Mediterranean Type Regions of the World. Council of Europe Publishing, Mèze, France. 

285 pp. 

CABI. 2013a. Crop Protection Compendium. Commonwealth Agricultural Bureau International 

(CABI). http://www.cabi.org/cpc/. 

CABI. 2013b. Invasive Species Compendium. Commonwealth Agricultural Bureau International 

(CABI). http://www.cabi.org/cpc/. 

Caton, B. P., A. L. Koop, L. Fowler, L. Newton, and L. Kohl. Quantitative uncertainty analysis 

for a weed risk assessment model. Risk Analysis. In Review. 
TP E-300, Ver. 2.3 February 11, 2019 51

Daehler, C. C., J. S. Denslow, S. Ansari, and H. C. Kuo. 2004. A risk-assessment system for 

screening out invasive pest plants from Hawaii and other Pacific Islands. Conservation 

Biology 18(2):360-368. 

Dave's Garden. 2014. Plant Files database. Dave's Garden. http://davesgarden.com/guides/pf/. 

DEPI. 2014. Victoria weed risk assessment (WRA). Department of Environment and Primary 

Industries (DEPI), Victoria, Australia. 

http://vro.depi.vic.gov.au/dpi/vro/vrosite.nsf/pages/weeds_vic_nox_review. 

Espenshade, E. B., Jr., J. C. Hudson, and J. L. Morrison (eds.). 1995. Goode's World Atlas (19th 

ed.). Rand McNally, U.S.A. 372 pp. 

eFloras. 2013. Electronic Floras, online database. Missouri Botanical Garden, St. Louis, Missouri 

and Harvard University Herbaria, Cambridge, Massachusetts, U.S.A. 

http://www.efloras.org. 

Foulis, L. 2001. The Plant Book: The World of Plants in a Single Volume. James Mills-Hicks. 

1019 pp. 

GardenWeb. 2014. GardenWeb: The Internet’s Largest Home and Garden Community. 

GardenWeb. http://www.gardenweb.com/. 

Gordon, D. R., D. A. Onderdonk, A. M. Fox, R. K. Stocker, and C. Gantz. 2008. Predicting 

invasive plants in Florida using the Australian weed risk assessment. Invasive Plant 

Science and Management 1:178-195. 

Holm, L., J. Doll, E. Holm, J. Rancho, and J. Herberger. 1997. World Weeds: Natural Histories 

and Distribution. John Wiley & Sons, Inc., New York, U.S.A. 1129 pp. 

IPPC, 1997. International Plant Protection Convention. Food and Agriculture Organization of the 

United Nations, Secretariat of the International Plant Protection Convention (IPPC), 

Rome, Italy. 18 pp. 

IPPC. 2011. International Standard For Phytosanitary Measures No. 2: Framework for Pest Risk 

Analysis. Food and Agriculture Organization of the United Nations, Secretariat of the 

International Plant Protection Convention (IPPC), Rome, Italy. 18 pp. 

IPPC. 2013a. International Standard For Phytosanitary Measures No. 5: Glossary of 

Phytosanitary Terms. Food and Agriculture Organization of the United Nations, 

Secretariat of the International Plant Protection Convention (IPPC), Rome, Italy. 35 pp. 

IPPC. 2013b. International Standard For Phytosanitary Measures No. 11: Pest Risk Analysis for 

Quarantine Pests. Food and Agriculture Organization of the United Nations, Secretariat 

of the International Plant Protection Convention (IPPC), Rome, Italy. 36 pp. 

ISSG. 2013. Global Invasive Species Database. International Union for Conservation of Nature 

(IUCN), Invasive Species Specialist Group (ISSG). 

http://www.issg.org/database/welcome/. 

Koop, A., L. Fowler, L. Newton, and B. Caton. 2012. Development and validation of a weed 

screening tool for the United States. Biological Invasions 14(2):273-294. 

Kowarik, I. 1995. Time lags in biological invasion with regard to the success and failure of alien 

species. Pages 15-38 in P. Pyšek, K. Prach, M. Rejmánek, and M. Wade, (ed.). Plant 

Invasions - General Aspects and Special Problems. SPB Academic Publishing, 

Amsterdam. 

Mack, R. N. 1996. Predicting the identity and fate of plant invaders: Emergent and emerging 

approaches. Biological Conservation 78:107-121. 
TP E-300, Ver. 2.3 February 11, 2019 52

NAPPO. 2008. NAPPO Regional Standards for Phytosanitary Measures (RSPM): Pest Risk 

Assessment for Plants for Planting as Quarantine Pests (RSPM No. 32). North American 

Plant Protection Organization (NAPPO), Ottawa, Ontario, Canada. 16 pp. 

Peel, M. C., B. L. Finlayson, and T. A. McMahon. 2007. Updated world map of the KöppenGeiger climate classification. Hydrology and Earth Systems Sciences 11:1633-1644. 

Pheloung, P. C., P. A. Williams, and S. R. Halloy. 1999. A weed risk assessment model for use 

as a biosecurity tool evaluating plant introductions. Journal of Environmental 

Management 57:239-251. 

Radosevich, S. R., and J. S. Holt. 1984. Weed Ecology: Implications for Vegetation 

Management. John Wiley & Sons, New York, U.S.A. 265 pp. 

Randall, J. M. 2007. The Introduced Flora of Australia and its Weed Status. CRC for Australian 

Weed Management, Department of Agriculture and Food, Western Australia, Australia. 

528 pp. 

Reichard, S. 2001. The search for patterns that enable prediction of invasion. Pages 10-19 in R. 

H. Groves, F. D. Panetta, and J. G. Virtue, (eds.). Weed Risk Assessment. CSIRO, 

Collingwood, Australia. 

Reichard, S. H., and C. W. Hamilton. 1997. Predicting invasions of woody plants introduced into 

North America. Conservation Biology 11(1):193-203. 

Richardson, D. M., P. Pyšek, M. Rejmánek, M. G. Barbour, F. D. Panetta, and C. J. West. 2000. 

Naturalization and invasion of alien plants: Concepts and definitions. Diversity and 

Distributions 6:93-107. 

Saha, S., S. Moorthi, H.-L. Pan, X. Wu, J. Wang, S. Nadiga, P. Tripp, R. Kistler, J. Woollen, D. 

Behringer, H. Liu, D. Stokes, R. Grumbine, G. Gayno, J. Wang, Y.-T. Hou, H.-y. 

Chuang, H.-M. H. Juang, J. Sela, M. Iredell, R. Treadon, D. Kleist, P. V. Delst, D. 

Keyser, J. Derber, M. Ek, J. Meng, H. Wei, R. Yang, S. Lord, H. v. d. Dool, A. Kumar, 

W. Wang, C. Long, M. Chelliah, Y. Xue, B. Huang, J.-K. Schemm, W. Ebisuzaki, R. Lin, 

P. Xie, M. Chen, S. Zhou, W. Higgins, C.-Z. Zou, Q. Liu, Y. Chen, Y. Han, L. Cucurull, 

R. W. Reynolds, G. Rutledge, and M. Goldberg. 2010. The NCEP climate forecast 

system reanalysis. Bulletin of the American Meteorological Society 91: 1015-1057. 

Space, J. C., and T. Flynn. 2002. Report to the Government of Samoa on Invasive Plant Species 

of Environmental Concern. United States Department of Agriculture, Forest Service, 

Honolulu, Hawaii, U.S.A. 80 pp. 

Univ. of California. 2014. Consortium of California Herbaria. Regents of the University of 

California. http://ucjeps.berkeley.edu/consortium/. 

Univ. of Florida. 2014. Herbarium Collections Catalog. University of Florida. 

http://www.flmnh.ufl.edu/herbarium/cat/catsearch.htm. 

Univ. of Hawaii. 2014. Weed Risk Assessments for Hawaii and Pacific Islands. University of 

Hawaii. http://www.botany.hawaii.edu/faculty/daehler/wra/default2.htm. 

Veitch, C. R., M. N. Clout, and D. R. Towns. 2011. Island Invasives: Eradication and 

Management. Proceedings of the International Conference on Island Invasives. 

International Union for Conservation of Nature (IUCN), Gland, Switzerland and The 

Centre for Biodiversity and Biosecurity (CBB), Auckland, New Zealand. xii + 542 pp. 

Verloove, F. 2006. Catalogue of Neophytes in Belgium (1800-2005). National Botanic Garden of 

Belgium, Meise, Belgium. 89 pp. 
TP E-300, Ver. 2.3 February 11, 2019 53

Waterhouse, D. F. 1993. The Major Arthropod Pests and Weeds of Agriculture in Southeast 

Asia. The Australian Centre for International Agricultural Research, Canberra, Australia. 

141 pp. 

Weber, E. 2003. Invasive Plant Species of the World: A Reference Guide to Environmental 

Weeds. CAB International (CABI) Publishing, Wallingford, U.K. 548 pp. 

Whitney, K. D., and C. A. Gabler. 2008. Rapid evolution in introduced species, 'invasive traits' 

and recipient communities: challenges for predicting invasive potential. Diversity and 

Distributions 14(4):569-580. 
TP E-300, Ver. 2.3 February 11, 2019 54

Appendices 

List of Appendices 

Appendix A. Questions and scoring used in the PPQ weed risk assessment. 

Appendix B. Logistic regression model formulas. 

Appendix C. Logistic regression model and decision thresholds for low risk, evaluate further, 

and high risk. 

Appendix D. Secondary screening tool of the PPQ WRA. 

Appendix E. Question-specific guidance and examples for the risk element 

Establishment/Spread Potential. 

Appendix F. Question-specific guidance and examples for the risk element Impact Potential. 

Appendix G. Question-specific guidance and examples for the risk element Geographic 

Potential. 

Appendix H. Question-specific guidance and examples for the risk element Entry Potential. 

Appendix I. Conceptual process for the uncertainty analysis in the PPQ WRA. 

Appendix J. Work instructions for performing an uncertainty analysis (a stochastic simulation) 

using @Risk. 
TP E-300, Ver. 2.3 February 11, 2019 55

Appendix A. Questions and scoring used in the PPQ weed risk assessment 

A1. Establishment/Spread Potential

Code Question 

ES-1 What is the taxon’s establishment and spread status outside its native range: (a) Introduced 

elsewhere =>75 years ago but not escaped; (b) Introduced <75 years ago but not escaped; (c) 

Never moved beyond its native range; (d) Escaped/Casual; (e) Naturalized; (f) Invasive; (?) 

Unknown. (a = -5, b = -2, c = 0, d = 0, e = 2, f = 5, ? = 0) 

ES-2 Is the species highly domesticated? (y = -3, n = 0, or ? = 0) 

ES-3 Are there any congeners that are considered significant weeds? (y = 1, n = 0, or ? = 0) 

ES-4 Is it shade tolerant at some stage of its life cycle? (y = 1, n = 0, or ? = 0) 

ES-5 Is the plant a vine or scrambling plant, or does it form tightly appressed basal rosettes? (y = 

1, n = 0, or ? = 0) 

ES-6 Does the taxon form dense thickets, patches, or populations? (y = 2, n = 0, or ? = 0) 

ES-7 Is the taxon an aquatic plant? (y = 1, n = 0, or ? = 0) 

ES-8 Is the taxon a grass (Poaceae)? (y = 1, n = 0, or ? = 0) 

ES-9 Is the taxon a nitrogen fixing woody plant? (y = 1, n = 0, or ? = 0) 

ES-10 Does the taxon produce viable seed or spores? (y = 1, n = -1, or ? = 0) 

ES-11 Is the taxon self-compatible or apomictic? (y = 1, n = -1, or ? = 0) 

ES-12 Does the taxon require specialist pollinators that are not present in the WRA area? (y = -1, n 

= 0, or ? = 0) 

ES-13 What is the taxon’s minimum generation time? Choose one: (a) less than a year with 

multiple generations per year; (b) 1 year, usually annuals; (c) 2 or 3 years; (d) more than 3 

years; or (?) unknown. (a = 2, b = 1, c = 0, d = -1, ? = 0) 

ES-14 Is the taxon a prolific seed producer? (y = 1, n = -1, or ? = 0) 

ES-15 Are propagules likely to be dispersed unintentionally by human activity? (y = 1, n = -1, or ? 

= 0) 

ES-16 Are propagules likely to be dispersed as contaminants or hitchhikers of any agricultural, 

forestry, or horticultural products? (y = 2, n = -1, or ? = 0) 

ES-17 What is the number of natural dispersal vectors as described by ES-17a through ES-17e? 

(none = -4, one = -2, two = 0, three = 2, four or five = 4) 

ES-17a Are propagules wind dispersed? (y, n, ?) 

ES-17b Are propagules water dispersed? (y, n, ?) 

ES-17c Are propagules dispersed by birds? (y, n, ?) 

ES-17d Are propagules dispersed by other animals (externally)? (y, n, ?) 

ES-17e Are propagules dispersed by other animals (internally)? (y, n, ?) 
TP E-300, Ver. 2.3 February 11, 2019 56

ES-18 Is there evidence that a persistent (>1 year) seed bank (or other propagules) is formed? (y = 

1, n = -1, or ? = 0) 

ES-19 Does the taxon tolerate or benefit from mutilation, cultivation, or fire? (y = 1, n = -1, or ? = 

0) 

ES-20 Does the taxon have resistance to some herbicides or demonstrate potential to acquire 

herbicide resistance? (y = 1, n = 0, or ? = 0) 

ES-21 Adaptive potential: Number of plant hardiness zones (out of 13) suitable for survival. (zero 

to three = -1, four to nine = 0, ten to thirteen = 1) 

ES-22 Adaptive potential: Number of Köppen-Geiger climate classes (out of 12) suitable for 

survival. (zero to two = -2, three = 0, four to twelve = 2) 

ES-23 Adaptive potential: Number of annual precipitation bands (out of 11) suitable for survival. 

(zero to four = -1, five to seven = 0, eight to eleven = 1) 

A2. Impact Potential

Code Question 

Imp-G1 Is the taxon allelopathic? (y = 0.1, n = 0, or ? = 0) 

Imp-G2 Is the taxon parasitic, and does it have potential hosts in the WRA area? (y = 0.1, n = 0, or ? 

= 0) 

Imp-N1 Does the taxon change ecosystem processes and parameters? (y = 0.4, n = 0, or ? = 0) 

Imp-N2 Does the taxon change habitat structure? (y = 0.2, n = 0, or ? = 0) 

Imp-N3 Does the taxon change species diversity? (y = 0.2, n = 0, or ? = 0) 

Imp-N4 Is the taxon likely to affect Federal Threatened & Endangered species? (y = 0.1, n = 0, or ? 

= 0) 

Imp-N5 Is the taxon likely to affect any U.S. globally outstanding ecoregion? (y = 0.1, n = 0, or ? = 

0) 

Imp-N6 What is the taxon’s weed status in natural systems? Choose one: (a) Taxon not a weed; (b) 

taxon a weed but no evidence of control; (c) taxon a weed and evidence of control efforts. 

(a = 0, b = 0.2, c = 0.6, ? = 0) 

Imp-A1 Does the taxon negatively impact personal property, human safety, or public infrastructure? 

(y = 0.1, n = 0, or ? = 0) 

Imp-A2 Does the taxon change or limit recreational use of an area? (y = 0.1, n = 0, or ? = 0) 

Imp-A3 Does the taxon affect desirable and ornamental plants and vegetation? (y = 0.1, n = 0, or ? = 

0) 

Imp-A4 What is the taxon’s weed status in anthropogenic systems? Choose one: (a) Taxon not a 

weed; (b) Taxon a weed but no evidence of control; (c) Taxon a weed and evidence of 

control efforts. (a = 0, b = 0.1, c = 0.4, or ? = 0) 

Imp-P1 Does the taxon reduce crop or commodity yield? (y = 0.4, n = 0, or ? = 0) 

Imp-P2 Does the taxon lower commodity value? (y = 0.2, n = 0, or ? = 0) 

Imp-P3 Does the taxon impact or is it likely to impact trade? (y = 0.2, n = 0, or ? = 0) 
TP E-300, Ver. 2.3 February 11, 2019 57

Imp-P4 Does the taxon affect the quality or availability of irrigation, or strongly compete with 

plants for water? (y = 0.1, n = 0, or ? = 0) 

Imp-P5 Is the taxon toxic to animals? (y = 0.1, n = 0, or ? = 0) 

Imp-P6 What is the taxon’s weed status in production systems? Choose one: (a) Taxon not a weed; 

(b) Taxon a weed but no evidence of control; (c) Taxon a weed and evidence of control 

efforts. (a = 0, b = 0.2, c = 0.6, ? = 0) 

A3. Geographic Potential 

For each of the three climate variables below, determine which hardiness zones, climate classes, or 

precipitation bands are suitable for species establishment. Select either y or n. 

Code Description 

Plant cold hardiness zones 

Geo-Z1 Zone 1 (below -50°F or below -45.6°C) 

Geo-Z2 Zone 2 (-50 to -40°F, or -45.6 to -40.0°C) 

Geo-Z3 Zone 3 (-40 to -30°F, or -40.0 to -34.4°C) 

Geo-Z4 Zone 4 (-30 to -20°F, or -34.4 to -28.9°C) 

Geo-Z5 Zone 5 (-20 to -10°F, or -28.9 to -23.3°C) 

Geo-Z6 Zone 6 (-10°F to 0°F, or -23.3 to -17.8°C) 

Geo-Z7 Zone 7 (0 to 10°F, or -17.8 to -12.2°C) 

Geo-Z8 Zone 8 (10 to 20°F, or -12.2 to -6.7°C) 

Geo-Z9 Zone 9 (20 to 30°F, or -6.7 to -1.1°C) 

Geo-Z10 Zone 10 (30 to 40°F, or -1.1 to 4.4°C) 

Geo-Z11 Zone 11 (40 to 50°F, or 4.4 to 10°C) 

Geo-Z12 Zone 12 (50 to 60°F, or 10 to 15.6°C) 

Geo-Z13 Zone 13 (above 60°F, or above 15.6°C) 

Köppen-Geiger climate classes 

Geo-C1 Tropical rainforest 

Geo-C2 Tropical savanna 

Geo-C3 Steppe 

Geo-C4 Desert 

Geo-C5 Mediterranean 

Geo-C6 Humid subtropical 

Geo-C7 Marine west coast 

Geo-C8 Humid continental warm summers 

Geo-C9 Humid continental cool summers 

Geo-C10 Subarctic 

Geo-C11 Tundra 

Geo-C12 Icecap 

10-inch precipitation bands (measurement in cm) 

Geo-R1 0-10 inches (0-25 cm) 

Geo-R2 10-20 inches (25-51 cm) 

Geo-R3 20-30 inches (51-76 cm) 

Geo-R4 30-40 inches (76-102 cm) 

Geo-R5 40-50 inches (102-127 cm) 

Geo-R6 50-60 inches (127-152 cm) 
TP E-300, Ver. 2.3 February 11, 2019 58

Geo-R7 60-70 inches (152-178 cm) 

Geo-R8 70-80 inches (178-203 cm) 

Geo-R9 80-90 inches (203-229 cm) 

Geo-R10 90-100 inches (229-254 cm) 

A4. Entry Potential

Code Question 

Ent-1 Is the plant taxon already in the United States? (y=1, n=0) STOP IF YES 

Ent-2 Is the plant taxon proposed for import or is its entry imminent (y=1, n=0) STOP IF YES 

Ent-3 What is the cultivation status of this plant taxon? Choose the best answer: (a) Neither 

cultivated or positively valued; (b) Not cultivated, but positively valued or potentially 

beneficial; (c) Cultivated, but no evidence of trade or resale; (d) Commercially cultivated or 

other evidence of trade or resale. (a = 0, b = 0.05, c = 0.25, d = 0.5) 

Ent-4 Entry as a contaminant 

Ent-4a Is the taxon present in Canada, Mexico, Central America, the Caribbean, or China? (y or n)5. 

Ent-4b Is the plant taxon a contaminant of propagative plant material (except seed)? (y = 0.4, n = 0, ? 

= 0) 

Ent-4c Is the plant taxon a contaminant of seeds for planting? (y = 0.4, n = 0, ? = 0) 

Ent-4d Is the plant taxon a contaminant of ballast? (y = 0.3, n = 0, ? = 0) 

Ent-4e Is the plant taxon a contaminant of aquarium plants or other aquarium products? (y = 0.2, n = 

0, ? = 0) 

Ent-4f Is the plant taxon a contaminant of imported landscape products? (y = 0.2, n = 0, ? = 0) 

Ent-4g Is the plant taxon a contaminant of containers, packing materials, equipment, conveyances, or 

non-agricultural products? (y = 0.2, n = 0, ? = 0) 

Ent-4h Is the plant taxon a contaminant of highly processed commodities or fruits, vegetables, grains 

or other products for consumption? (y = 0.1, n = 0, ? = 0) 

Ent-4i Is the plant taxon a contaminant of some other pathway(s) not covered above? Choose the 

letter for the risk score that best represents the relative risk for the pathway(s): (a) 0; (b) 0.01; 

(c) 0.02; (d) 0.03; (e) 0.04. 

Ent-5 Is the plant taxon likely to enter the United States through natural dispersal? (y = 0.6, n = 0, ? 

= 0) 

 

 

5

 This question is not scored directly. If the taxon is present in one of these regions or countries, then the scores for 

questions Ent-4b through Ent-4i are doubled. 
TP E-300, Ver. 2.3 February 11, 2019 59

Appendix B. Logistic regression model formulas 

This appendix contains the formulas for the logistic regression model of the probabilities 

(P) of a taxon being a major-invader (MajI), minor-invader (MinI), and non-invader 

(NonI). In this usage, an invader is broadly defined to mean a species capable of 

establishing, spreading, and causing impacts in a broad range of environmental, 

production, and anthropogenic systems. ES and Imp refer to the risk scores from the 

Establishment/Spread and Impact Potential risk elements, respectively. The three 

probabilities sum to 1 for each plant [P(MajI) +P(MinI) +P (NonI) =1]. These 

probabilities are automatically calculated by the Excel workbook in cells C139 to C141 

on the “Screening Tool” worksheet. For additional information on the model or its 

development, see Koop et al. (2012). 

ܲሺܯ݆ܽܫሻ ൌ 	

1

1൅݁൫ସ.ଵଷସ଼	ି	ሺ଴.ଶଷହ଺ൈாௌሻିሺ଴.଺଴ଵଽൈூ௠௣ሻ൯

ܲሺܯ݅݊ܫሻ ൌ 	

1

1൅݁൫଴.଺ଷ଺଺	ି	ሺ଴.ଶଷହ଺ൈாௌሻିሺ଴.଺଴ଵଽൈூ௠௣ሻ൯

ሻܫ݆ܽܯሺܲ	– 

ܲሺܰ݋݊ܫሻ ൌ1െ൬ 1

1൅݁൫଴.଺ଷ଺଺	ି	ሺ଴.ଶଷହ଺ൈாௌሻିሺ଴.଺଴ଵଽൈூ௠௣ሻ൯

൰

 
TP E-300, Ver. 2.3 February 11, 2019 60

Appendix C. Logistic regression model and decision thresholds for Low Risk, 

Evaluate Further, and High Risk 

In this appendix we present the logistic regression model for the probabilities of being a noninvader (dotted line), minor-invader (dashed line), and major-invader (solid line) as a function of 

the composite risk score. Composite risk score refers to a linear combination of the risk scores 

for the Establishment/Spread (ES) and Impact (Imp) Potential risk elements. It is used in 

determining the probabilities described in Appendix B and is calculated as (0.2356 × ES) + 

(0.6019 × Imp) (Koop et al., 2012). The vertical lines are the cutoff scores used by the model to 

determine the outcome of the WRA (Low Risk, Evaluate Further, or High Risk). The cutoff 

scores were calculated by receiver operating characteristic (ROC) curve analysis (Koop et al., 

2012). This analysis maximizes the probabilities of accurately identifying non- and majorinvaders, while minimizing errors. In the analysis, we assumed that the cost of a false-positive 

and false-negative error were equal. If the probability of being a non-invader is ≥ 0.449 

(occurring when the composite risk score ≤ 0.841), then the species is classified as Low Risk. If 

the probability of being a major-invader is ≥ 0.388 (occurring when the composite risk score ≥ 

3.769), then the species is classified as High Risk. All other species are classified as Evaluate 

Further pending secondary screening. For additional information on the development of the 

model see Koop et al. (2012). 

0

0.2

0.4

0.6

0.8

1

-4 -3 -2 -1 0 1 2 3 4 5 6 7 8

Probability / Percentage

Composite Risk Score

Low Risk

P(Non-Invader) ≥ 0.449

Evaluate 

Further

High Risk

P(Major-Invader) ≥ 0.388
TP E-300, Ver. 2.3 February 11, 2019 61

Appendix D. Secondary screening tool of the PPQ WRA 

The secondary screening tool uses several questions that are strongly predictive of weed risk to 

determine if some of the taxa classified by the primary model as Evaluate Further should be 

categorized as High Risk or Low Risk. The first is question ES-1 from the WRA model, which 

refers to the species’ invasive status anywhere in the world, including in the United States. The 

left and right paths correspond to two of the status options from ES-1. The central and bottom 

diamonds incorporate two other status options from ES-1. In those diamonds, the secondary 

score is the sum of the scores for six questions from the WRA model: 1) prolific reproduction 

(ES-14), 2) minimum generation time (ES-13), 3) shade adapted (ES-4), 4) commodity 

contaminant (ES-16), 5) number of natural dispersal vectors (ES17), and 6) forms dense thickets 

(ES-6). 
TP E-300, Ver. 2.3 February 11, 2019 62

 

What is its 

invasive status? 

High Risk Low Risk 

Evaluate 

Further

Introduced > 75 years 

ago and never has 

escaped 

Invasive (spreading or 

has spread throughout 

a region) 

Naturalized 

somewhere 

and 2° score > 

0? 

Neither 

Introduced < 75 

years ago, never 

escaped, and 2° 

score < 0? 

No 

Yes 

Yes 

No 
TP E-300, Ver. 2.3 February 11, 2019 63

Appendix E. Question-specific guidance and examples for the risk element 

Establishment/Spread Potential 

The Establishment/Spread Potential risk element evaluates the likelihood the taxon will escape, 

naturalize, and spread in the area at risk by evaluating numerous factors about the biology of the 

taxon. It does not consider any evidence about the impact a taxon is or is not causing. The risk 

score may range from -25 to 32 points, with higher numbers indicating a higher likelihood. 

Questions usually require an answer of yes (y), no (n), or unknown (?), except where otherwise 

indicated. 

For each question, we provide the question, answer choices, and scoring in bold type followed by 

the question-specific guidance. As necessary, additional suggestions, definitions, search criteria, 

and references are provided. One or more examples are also provided. For the examples, the full 

citations for references are not provided. This omission is intentional. 

ES-1 What is the taxon’s establishment and spread status outside its native range: (a) 

Introduced elsewhere =>75 years ago but not escaped; (b) Introduced <75 years ago but 

not escaped; (c) Never moved beyond its native range; (d) Escaped/Casual; (e) Naturalized; 

(f) Invasive; (?) Unknown. (a = -5, b = -2, c = 0, d = 0, e = 2, f = 5, ? = 0) 

Definitions 

 Casual: Alien plants “that may flourish and even reproduce occasionally in an area, but 

which do not form self-replacing populations, and which rely on repeated introductions 

for their persistence.” These taxa are typically called waifs, transients, and occasional 

escapes, or they may be described as “persisting after cultivation” in the literature 

(Richardson et al., 2000). Crop plants that appear a season or two after they were planted 

but do not persist are called volunteers (Gressel, 2005) and are considered casuals in the 

context of the PPQ WRA. 

 Naturalized: “Alien plants that reproduce consistently and sustain populations over many 

life cycles without direct intervention by humans; they often recruit offspring freely, 

usually close to adult plants, and do not necessarily invade natural, semi-natural or 

human-made ecosystems” (Richardson et al., 2000). Many sources use the word 

“established” as a synonym of “naturalized”; however, other sources consider 

“established” to refer to plants that are not quite fully naturalized (i.e., surviving, but not 

reproducing) (Richardson et al., 2000). 

 Invasive: “Naturalized plants that produce offspring, often in very large numbers, at 

considerable distances from parent plants, and thus have the potential to spread over a 

considerable area” (Richardson et al., 2000). Consider “considerable distances” to be 

distances of more than 100 meters for taxa spreading by seeds and other propagules, and 

distances of more than 6 meters every three years for taxa spreading by roots, rhizomes, 

stolons, or creeping stems (Richardson et al., 2000). Words such as “spreading” and 

“expanding” are indicative of an invasive species, in the strict sense. Naturalized exotic 

species that are widely distributed across an area are also likely to be invasive or have 

been invasive in the past. 
TP E-300, Ver. 2.3 February 11, 2019 64

Rationale. One of the best predictors of invasiveness (in the strict sense) is a taxon’s status where 

it has been introduced outside of its native range (Caley and Kuhnert, 2006; Reichard, 2001). 

Review the literature to determine the invasive status of the taxon around the world (see 

definitions below). 

Criteria. If the taxon is relatively new (e.g., recently bred, artificial hybrid, genetically modified 

organism, new natural hybrid) and has not had sufficient time to express its potential 

invasiveness (or lack of it) under uncontrolled conditions answer unknown. Because the invasive 

plant literature uses conflicting terminology to describe a taxon’s status, carefully evaluate the 

evidence by either reading the source’s definitions of its terminology or examining the 

underlying supporting evidence. For example, species described as “widely naturalized 

throughout the country” or “rapidly spreading” should be considered invasive, unless the 

evidence suggests otherwise. For the terms listed in choices “d,” “e,” and “f” we follow 

Richardson et al.’s (2000) concepts and definitions (see above). For this question, it is not 

appropriate to use congeneric information. 

We recommend that you begin your compiled evidence with a single sentence describing the 

native distribution. Then describe the status of the species for the countries where it has been 

introduced. This list does not need to be exhaustive, just representative. Be sure to provide your 

reasoning or supporting documentation for every country/case as necessary. Because a taxon’s 

status may vary depending on what country it is in, choose the answer that describes its worst 

status. In other words, if a taxon has not escaped in a country where it is cultivated, but it has 

become naturalized in another country, choose “e.” If a species is invasive in one country and 

naturalized in five others, choose “f.” Evidence that a plant is a weed in a given country is almost 

always inappropriate for this question and risk element because weediness generally refers to 

harmful or unwanted plants; this type of evidence is considered under Impact Potential. 

Notes 

 If a taxon has been introduced elsewhere, but you do not know when it was introduced, 

assume that it was recently introduced (choice “b”) but state in “Evidence and Notes” that 

you are making this assumption. When available, provide dates of introduction. 

 Although we use the term “invasive” to describe the class of plants under choice “f,” 

minimize or avoid the use of this term when presenting and describing the evidence for 

any given country. In the scientific and popular literature, the term “invasive” can refer 

partially or entirely to plants that are harmful and unwanted. Statements in the evidence 

that a taxon is “invasive” in a given country are generally too ambiguous for this 

question. However, evidence that a taxon is “invading” an area may be appropriate as it 

implies an ecological process rather than a perceived status. To avoid confusion, we 

recommend that you describe the evidence for a taxon meeting the criteria for choice “f” 

in terms of its ability to spread. 

Full examples 

 Gladiolus undulatus is native to South Africa (Goldblatt and Manning, 1998). It is 

naturalized in Portugal (Domingues de Almeida and Freitas, 2006). It “is increasing very 

rapidly on road verges, creek banks, wetlands and estuarine sites in bushland in 

Australia” (Hussey et al., 2007). It invades grassland, forest edges, riparian habitats, 
TP E-300, Ver. 2.3 February 11, 2019 65

seasonal freshwater wetlands (Weber, 2003). The alternate answers for the stochastic 

simulation were both “e.” Answer: “f” with low uncertainty. 

References 

 Caley, P., and P. M. Kuhnert. 2006. Application and evaluation of classification trees for 

screening unwanted plants. Austral Ecology 31(5):647-655. 

 Gressel, J. (ed.). 2005. Crop Ferality and Volunteerism. CRC Press, Boca Raton, FL. 422 

pp. 

 Reichard, S. 2001. The search for patterns that enable prediction of invasion. Pages 10-19 

in R. H. Groves, F. D. Panetta, and J. G. Virtue, (eds.). Weed Risk Assessment. CSIRO, 

Collingwood, Australia. 

 Richardson, D. M., P. Pyšek, M. Rejmánek, M. G. Barbour, F. D. Panetta, and C. J. West. 

2000. Naturalization and invasion of alien plants: Concepts and definitions. Diversity and 

Distributions 6:93-107. 

ES-2 Is the species highly domesticated? (y = -3, n = 0, or ? = 0)? 

Criteria. This question will rarely receive a positive answer. Answer yes if the taxon has been 

intentionally selected over several to many generations for a particular trait or suite of traits that 

likely reduces the ability of the plant to escape, establish, and spread (e.g., much reduced flower 

or seed production, reduced vigor, loss of vegetative growth, conversion of stamens to petals). 

An answer of yes should be accompanied by evidence that people substantially modified one or 

more traits through domestication (traditional breeding, biotechnology, etc.) and that the 

modification applies to all lower level taxa including cultivars. Evidence to the contrary [no 

domestication, or selection that increases invasive traits (e.g., Kitajima et al., 2006)] or no 

information results in a no response. Breeding that has changed neutral traits (i.e., those not 

affecting plant invasive potential) results in a no response. 

Notes 

 We recommend, for all taxa and scenarios, that you describe whether or not the taxon is 

cultivated, because by definition non-cultivated species cannot be domesticated. If a 

taxon is not cultivated, use negligible uncertainty. 

Full examples 

 Cordia curassavica is cultivated as a medicinal plant in Brazil (Albuquerque et al., 2012; 

Mussi-Dias et al., 2012). It is used in traditional Mexican medicine (Hernandez et al., 

2007). However, we found no evidence that it has been bred for any particular traits 

resulting in reduced weed potential. Answer: no with low uncertainty. 

 Fortunella japonica (kumquat) is cultivated for its fruit. This taxon is “known only from 

cultivation” (Ziegler and Wolfe, 2008). Some varieties are seedless (Morton, 1987). 

“Kumquats are rarely grown from seed as they do not do well on their own roots” 

(Morton, 1987). Seedlings do poorly on their own (Verheij and Coronel, 1991). Based on 

this suite of traits and the fact this species is only known from cultivation, domestication 

appears to have reduced the overall ability of the species to survive on its own. Answer: 

yes with low uncertainty. 
TP E-300, Ver. 2.3 February 11, 2019 66

References 

 Kitajima, K., A. M. Fox, T. Sato, and D. Nagamatsu. 2006. Cultivar selection prior to 

introduction may increase invasiveness: evidence from Ardisia crenata. Biological 

Invasions 8:1471-1482. 

ES-3 Are there any congeners that are considered significant weeds? (y = 1, n = 0, or ? = 0) 

Criteria. Review major works and databases to determine if any congeners (taxa within the same 

genus) are considered significant weeds, meaning those taxa that cause significant direct or 

indirect damage to production, natural, or anthropogenic systems, or that result in high economic 

costs due to control efforts. Significant weeds are often subject to some form of control or 

management to limit their impacts. Taxa classified as serious or principal in Holm et al. (1979) 

or troublesome in Bridges (1992) should be considered significant weeds. Similarly, plants 

considered invasive or highly invasive (as defined under ES-1) in the literature may also be 

significant weeds [e.g., plants listed under category #5 in Randall (2007), or plants listed in other 

works on major world invaders such as Weber (2003)]. Taxa that are regulated by national plant 

protection organizations may also qualify as significant weeds. If no weedy congeners are found, 

answer no. Remember that only congeners that are significant weeds merit a yes. 

Notes 

 Searching: Genus name + “weed,” “invas*,” “invad*,” “pest.” Evidence is usually 

obtained from internet, primary literature, and weed handbooks [e.g., Weber (2003)]. 

 Be cautious about relying on a single source when evaluating if a taxon is a significant 

weed because some sources exaggerate impacts or claim a taxon is a significant weed 

without provide clear evidence. Remember, you are looking to see if the genus has any 

significant weeds as an indication that the taxon being assessed may also be 

weedy/invasive. 

 For perspective, we recommend stating the approximate number of taxa contained in the 

genus. Mabberley (2008) reports taxa numbers for each plant genus. 

 Usually, congeners all have a similar life form (e.g., herbs, shrubs, trees, vines, aquatics, 

etc.). For genera with a diverse set of life forms or very divergent life histories, we 

recommend you limit the comparison to those taxa with similar life form and biology. 

 In some cases, the genus may contain only one taxon. In these cases, if the taxon being 

assessed is closely related to members of another genus, or was recently classified in that 

genus, expanding the scope of this question to that genus may be appropriate. However, 

state why this is being done, and limit the comparison to similar taxa. 

 Use the current information on taxonomy and nomenclature [e.g., the GRIN database 

(NGRP)] to determine which taxa are within the same genus. Some consideration may be 

given to taxa that were previously considered to be in the same genus since they will 

likely represent similar species (see full example below); but use this approach only when 

no other information is available. 

Full examples 

 Delairea odorata is the only species in the genus Delairea (Mabberley, 2008), but if we 

include species in the genus Senecio, in which D. odorata was once placed, several 

significant congeneric weeds exist (Holm et al., 1979; Randall, 2007; Weber, 2003). For 
TP E-300, Ver. 2.3 February 11, 2019 67

example, S. jacobaea is highly toxic to livestock (Bossard et al., 2000), and S. vulgaris is 

a serious and principal weed in many countries (Holm et al., 1979). Senecio angulatus

changes community structure, alters species composition (Newton, 1996; Weber, 2003; 

WMC, 2013), and reduces regeneration of native species (Williams and Hayes, 2007). 

Because these taxa are not truly congeners of D. odorata, we answered yes but with 

moderate uncertainty. Answer: yes with moderate uncertainty. 

References 

 Bridges, D. C. (ed.). 1992. Crop Losses Due to Weeds in Canada and The United States - 

1992. Weed Science Society of America, Champaign, IL, U.S.A. 403 pp. 

 Holm, L. G., J. V. Pancho, J. P. Herberger, and D. L. Plucknett. 1979. A Geographical 

Atlas of World Weeds. Krieger Publishing Company, Malabar, Florida, U.S.A. 391 pp. 

 Mabberley, D. J. 2008. Mabberley's Plant-Book: A Portable Dictionary of Plants, their 

Classification and Uses (3rd edition). Cambridge University Press, New York. 1021 pp. 

 NGRP. Germplasm Resources Information Network (GRIN). United States Department 

of Agriculture, Agricultural Research Service, National Genetic Resources Program 

(NGRP). http://www.ars-grin.gov/cgi-bin/npgs/html/index.pl?language=en.

 Randall, J. M. 2007. The Introduced Flora of Australia and its Weed Status. CRC for 

Australian Weed Management, Department of Agriculture and Food, Western Australia, 

Australia. 528 pp. 

 Weber, E. 2003. Invasive Plant Species of the World: A Reference Guide to 

Environmental Weeds. CABI Publishing, Wallingford, UK. 548 pp. 

ES-4 Is it shade tolerant at some stage of its life cycle? (y = 1, n = 0, or ? = 0) 

Criteria. Answer yes if you find evidence that the taxon can grow in full shade (low light levels 

and no direct light) at any stage in the life cycle. Also answer yes for submerged aquatic taxa 

unless contrary information is available. Answer no if the taxon requires full sun or partial 

sun/partial shade at all stages of its life cycle; otherwise, answer unknown. If quantitative data 

are available, choose the shade tolerance status supported by the data; otherwise, consider the 

weight of the evidence. Consideration of the taxon’s ecological niche may also help address this 

question (e.g., taxa that require canopy disturbance are often intolerant of shade, whereas forest 

understory taxa are often shade tolerant). 

Notes 

 With respect to percentage of full light availability, there is no strict criterion for shade. 

However, for the purpose of this question we will consider 20 percent of full sunlight or 

less to represent shaded conditions. For reference, we note that in some tropical forests 

only 2 percent of available sunlight reaches the forest floor. 

 A study of light intensity in natural lakes in Wisconsin showed that plants at a depth of 5 

meters receive anywhere between 17 percent to less than 1 percent of incident light due 

to light attenuation in the water column (Riemer, 1993). Plants that grow to these depths 

should receive a yes with higher uncertainty if more direct evidence is not available. 

 Information about a taxon’s behavior or traits can also be used to answer this question 

(e.g., light saturation curves, phototropism, leaf movements, etc.). 
TP E-300, Ver. 2.3 February 11, 2019 68

 Gardening guides (e.g., http://davesgarden.com/) usually provide some indication of a 

taxon’s light preferences; however, these guides may not always indicate light tolerances 

under natural conditions. 

 The CABI Forestry Compendium (http://www.cabi.org/fc/) also gives shade tolerance 

information for a number of woody taxa. 

Full examples 

 Impatiens parviflora prefers shade and half-shade (Valkenburg and Duistermaat, 2012). It 

occurs in shady places (Groom, 2012; Perglová et al., 2009; Salisbury, 1961; Valkenburg 

and Duistermaat, 2012). In its introduced range, “[m]ostly it is found in damp shady 

places“ (Williamson, 1996). “Its ability to grow and reproduce in low light levels is 

considered the main factor enabling its spread into forests” (Tabak and Von Wettberg, 

2008). In Canada, it grows “in even the deepest forest shade” (DAF, 2011). Answer: yes 

with negligible uncertainty. 

 Gladiolus undulatus is “not shade tolerant” (Auckland Council, 2007). It requires full sun 

to partial shade (Dave's Garden, 2011). Answer: no with low uncertainty. 

References 

 Riemer, D. N. 1993. Introduction to Freshwater Vegetation. Krieger, Malabar, Florida. 

205 pp. 

ES-5 Is the plant a vine or scrambling plant, or does it form tightly appressed basal 

rosettes? (y = 1, n = 0, or ? = 0) 

Rationale. Taxa with these life forms typically readily grow over and smother existing 

vegetation, possibly suppressing their growth or eventually killing them. 

Criteria. Vines and other plants with a scrambling/clambering life form receive a yes. Plants with 

basal rosettes that are flat, tightly appressed and with overlapping leaves receive a yes as well. 

Answer no for taxa that do not physically overgrow other vegetation in this manner. Plants such 

as trees that simply grow taller and shade out underlying vegetation fall outside of the scope of 

this question. Information on a taxon’s growth form is usually readily available, particularly in 

floras; however, if it is not, answer unknown. This question may overlap with question ES-6, 

which relates to plants forming dense populations. Question ES-6 focuses on population density 

and how that impacts other organisms, whereas this question focuses on the growth form/habit of 

an individual plant. Certain aquatics or terrestrial herbs may receive a yes if individual plants 

form mats, although this may be covered under ES-6 because mat-forming plants often root at 

nodes resulting in a “population” of plants that are interconnected. 

Notes 

 The evidence should describe the taxon’s life form. Usually this will be sufficient to 

answer the question, but in some cases further discussion and interpretation may be 

necessary. 

 Scrambling plants (also known as scandent, sprawling, or clambering) are taxa that often 

use stiff branches or hooks to climb and clamber over other vegetation. They differ from 
TP E-300, Ver. 2.3 February 11, 2019 69

other plants that use more active mechanisms of climbing such twining, adventitious 

roots, or tendrils. 

Full examples 

 Impatiens parviflora is neither a vine nor an herb with a basal rosette. It is an erect annual 

herb, 20-150 cm tall (Coombe, 1956). It is a tall herb (Wallentinus, 2002), the stem being 

erect and simple or branched, and the height of the plant being 0.3 to 1.5 meters 

(Valkenburg and Duistermaat, 2012). Answer: no with negligible uncertainty. 

 Persicaria chinensis is a vine (Biosecurity New Zealand, 2011c). Its stems are “often 

very long and climbing” (PROSEA Foundation, 2011; van Valkenburg and 

Bunyapraphatsara, 2001). It is a “hardy creeper” (Barbora, 1972). It “can grow rapidly to 

varying heights depending on what it is scrambling over” (Galloway and Lepper, 2010). 

It “can scramble up trees to about 10 m” (Goodland and Healey, 1996). It climbs over tea 

bushes (Haridas and Sharma, 1974). It is a “vigorous climber with the ability to smother 

native plants, forest areas and horticulture operations” (Biosecurity New Zealand, 2011c). 

Answer: yes with negligible uncertainty. 

ES-6 Does the taxon form dense thickets, patches, or populations? (y = 2, n = 0, or ? = 0) 

Rationale. Taxa capable of dense growth may be good invaders because they may suffer less 

from intraspecific competition than other taxa and may exclude other taxa or obstruct access as 

they dominate an area. Such taxa may better establish in undisturbed habitats where competition 

is high. 

Criteria. To answer this question with a yes response, look for evidence that the taxon forms 

dense thickets, patches, or populations. Densely growing woody perennials and tall grasses are 

most likely to receive yes answers, but other life forms, including aquatics, may receive a yes. 

Taxa that merely have a dense growth form (i.e., stems and leaves arranged thickly on an 

individual plant) do not merit a yes on that basis alone. Instead, groups of individuals must be 

growing together in dense patches or populations. Answer no if there is no evidence that it forms 

dense thickets, patches, or populations; otherwise, answer unknown as appropriate. Evidence for 

this question could conceivably also apply to Imp-N3, which concerns a plant changing 

community composition or reducing biodiversity because it forms monospecific populations, 

which are often dense. To ensure proper use, a “yes” response for this question should be based 

on finding positive evidence that a plant forms dense thickets, patches, or populations, and not

that it reduces biodiversity or smothers others vegetation. 

Notes 

 Check descriptions of growth and images of plant populations. You can use images as 

evidence when they are of natural or wild populations (as opposed to domestic plantings). 

Full examples 

 Arundo donax produces many-stemmed tussocks (Weber 2003). It forms “large, 

continuous, clonal root masses, sometimes covering several acres” (GISD 2011). The 

Global Invasive Species Database (2011) also refers to “dense populations.” “In Hawaii, 
TP E-300, Ver. 2.3 February 11, 2019 70

it has ‘naturalised in coastal areas, often in thickets’ (Wagner et al. 1999)” (in Csurhes 

2009). Answer: yes with negligible uncertainty. 

 Chrysopogon aciculatus forms “dense green mats...It spreads and forms a firm mat over

the ground” (Galinato et al., 1999). “An extensively creeping perennial with many rather 

brittle, leafy stolons rooting at the joints and forming a close, thick mat” (Whitney et al., 

1939). “Creeping perennial grass forming mats by means of leafy stolons...leaves [of 

culms] mostly crowded near the base” (Whistler, 1995). Mat-forming grass (ABRS, 

2012; Whistler, 1995). Answer: yes with negligible uncertainty.

ES-7 Is the taxon an aquatic plant? (y = 1, n = 0, or ? = 0) 

Rationale. Aquatic weeds are a major concern because they often choke waterways, hinder 

recreation and navigation, and alter light, oxygen, and nutrient levels. 

Criteria. Determining if a particular taxon is an aquatic species is sometimes difficult because 

there is a continuum of plant adaptations between terrestrial and aquatic environments. Answer 

yes to any obligate aquatic taxa that are floating, emergent, or submerged plants in fresh or 

saltwater systems. Wetland taxa and those that only grow on stream banks do not qualify. In 

difficult cases, consider the overall distribution of the taxon (e.g., where does it reproduce?), or 

note specific adaptations it may have for an aquatic habitat. A lack of positive evidence for this 

question results in a no answer. 

Notes 

 Most responses will be accompanied by either negligible or low uncertainty, but for some 

ambiguous taxa, uncertainty may be high. 

Full examples 

 Limnobium laevigatum is a free-floating or rooted aquatic with interconnected rosettes 

(Acevedo-Rodríguez and Strong, 2005). Limnobium seeds germinate underwater (Cook 

and Urmi-König, 1983). Answer: yes with negligible uncertainty.

 “Iris pseudacorus usually grows in sites with a continuously high soil-water content but 

the soil does not need to be submerged and the plant is capable of growth in dry sandy 

soil” (Sutherland, 1990). Iris pseudacorus occurs at higher positions along the shore of 

riparian zones and seeds germinate on exposed soil (Coops and Van Der Velde, 1995). It 

is “usually found near streams” (James, 1956). The rhizomes can grow when submerged 

in water 25 cm deep (Sutherland and Walton, 1990). Rhizomes continue to grow even 

after three months without water (Ramey and Peichel, 2001; Sutherland, 1990). 

“[C]onsidered as [an] emergent aquatic [weed]” (Gedebo and Froud-Williams, 1998). 

Because this plant is able to grow in dry soil and is not an obligate aquatic species, we 

answered no. Answer: no with moderate uncertainty. 

References 

 Pieterse, A. H., and K. J. Murphy. 1990. Aquatic Weeds: The Ecology and Management 

of Nuisance Aquatic Vegetation. Oxford University Press, New York. 593 pp. 
TP E-300, Ver. 2.3 February 11, 2019 71

ES-8 Is the taxon a grass (Poaceae)? (y = 1, n = 0, or ? = 0) 

Criteria. Answer yes for all taxa in the Poaceae, including bamboos. Otherwise, answer no. 

Notes 

 Sedges and rushes are technically not grasses and should not receive a yes. 

 Unless taxonomic issues make it difficult to determine if the taxon is in the Poaceae, 

uncertainty should always be negligible. 

Full examples 

 Cordia curassavica is not a grass; rather it is a shrub in Boraginaceae family (NGRP, 

2013). Answer: no with negligible uncertainty. 

ES-9 Is the taxon a nitrogen-fixing woody plant? (y = 1, n = 0, or ? = 0) 

Rationale. A large proportion of woody legumes are weeds, particularly in conservation areas, 

because they alter soil nitrogen levels, which in turn affects other species. In some cases, such 

plants may alter the successional dynamics of plant communities (e.g., Myrica faya in Hawaii; 

Vitousek et al., 1987). This question focuses on woody nitrogen-fixing taxa because they appear 

to have a potentially stronger impact on soil nitrogen (e.g., Yelenik et al, 2007). 

Criteria. Assume that all woody taxa of the family Fabaceae fix nitrogen (i.e., answer yes) unless 

you find evidence that a particular taxon does not. Answer no for all herbaceous or semi-woody 

taxa of the Fabaceae family, regardless of whether or not they fix nitrogen. Based on the 

resources listed below, ten angiosperm families are known to contain nitrogen-fixing taxa. If the 

taxon being assessed occurs in one of these families, look for specific evidence of nitrogen 

fixation and answer accordingly. If the taxon does not occur in one of these families, then answer 

no with negligible uncertainty. 

Notes 

 Search taxon name + “fix” and “nitrogen.” 

 In addition to the Fabaceae, the following families contain nitrogen-fixing woody plants: 

Betulaceae, Cannabaceae (Parasponia spp.), Casuarinaceae, Coriariaceae, Datiscaceae, 

Elaeagnaceae, Myricaceae, Rhamnaceae, Rosaceae, and Ulmaceae (Martin and Dowd, 

1990; Santi et al., 2013). Note that species in other plant families may rarely fix nitrogen; 

those should become apparent during a routine search of the literature. 

 The following website lists some angiosperm taxa that form symbiotic relationships with 

the nitrogen-fixing bacteria Frankia: 

http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm 

 Specific evidence that a given taxon fixes nitrogen results in lower uncertainty. If it is not 

clear whether the taxon is woody or not, raise uncertainty as appropriate. 

Full examples 

 We found no evidence that Cordia curassavica fixes nitrogen. Furthermore, it is in the 

Boraginaceae (NGRP, 2013), which is not one of the families known to contain nitrogen-
TP E-300, Ver. 2.3 February 11, 2019 72

fixing species (Martin and Dowd, 1990; Santi et al., 2013). Answer: no with negligible 

uncertainty. 

 Colophospermum mopane is a woody plant in the Fabaceae family (NGRP, 2013; Royal 

Botanic Gardens Kew, 2013), but evidence indicates that it does not fix nitrogen. In a 

study evaluating multiple tree species, C. mopane was one of the non-nitrogen fixing 

species (Pokhriyal et al., 1990). Nitrogen-fixing nodules have not been found (Melusi and 

Mojeremane, 2012). It “does not have symbiotic nitrogen-fixing rhizobium within its 

roots, i.e. it does not fix nitrogen” (cited in Mutakela, 2009). Because this woody plant is 

in the family Fabaceae and we found only one study that specifically looked at whether 

C. mopane fixes nitrogen, we used low instead of negligible uncertainty. Answer: no with 

low uncertainty. 

References 

 Martin, P. G., and J. M. Dowd. 1990. A protein sequence study of the dicotyledons and 

its relevance to the evolution of the legumes and nitrogen fixation. Australian Systematic 

Botany 3:91-100. 

 Santi, C., D. Bogusz, and C. Franche. 2013. Biological nitrogen fixation in non-legume 

plants. Annals of Botany 111(5):743-767. 

 Vitousek, P. M., L. R. Walker, L. D. Whiteaker, D. Mueller-Dombois, and P. A. Matson. 

1987. Biological invasion by Myrica faya alters ecosystem development in Hawaii. 

Science 238(4828):802-804. 

 Yelenik, S. G., W. D. Stock, and D. M. Richardson. 2007. Functional group identity does 

not predict invader impacts: Differential effects of nitrogen-fixing exotic plants on 

ecosystem function. Biological Invasions 9:117-125. 

ES-10 Does the taxon produce viable seed or spores? (y = 1, n = -1, or ? = 0) 

Criteria. Evidence that the taxon produces viable seed or spores naturally, either in the wild or in 

a garden (native or naturalized range), results in a yes answer. Information from gardening 

sources indicating that the plant is propagated by seed may be used to support a yes answer, but 

use higher uncertainty if you do not find evidence that it produces seed naturally. Taxa that can 

only be propagated through hand-pollination do not warrant a yes answer. If the taxon is a 

subspecies, variety, or cultivar, it must be indisputably sterile, not just self-incompatible, to 

receive a no answer (c.f. Culley and Hardiman, 2009, where ‘Bradford’ pear is self-sterile but in 

fact will cross with other cultivars to produce viable seed). Any taxon receiving a no answer for 

this question will also score no for question ES-14 (prolific seed production). Answer unknown 

if you find no information on seed or spore viability. 

Notes 

 Search taxon name + “seedling,” “seed,” “spore,” “germination,” and “germinate.” 

 Data on seed viability for >10,000 taxa exist at the Kew Gardens Seed Information 

Database (http://data.kew.org/sid/), but the source literature should be checked. 

 If seed or spore production is rare or occurs at very low rates, answer yes, but document 

that it is rare. Taxa with rare seed or spore production will be distinguished from taxa 

with prolific reproduction by question ES-14. 
TP E-300, Ver. 2.3 February 11, 2019 73

References 

 Culley, T., and N. Hardiman. 2009. The role of intraspecific hybridization in the 

evolution of invasiveness: a case study of the ornamental pear tree, Pyrus calleryana. 

Biological Invasions 11(5):1107-1119. 

Full examples 

 Ardisia crenata produces viable seeds (Chimera and Drake, 2010; Csurhes and Edwards, 

1998; Langeland and Burkes, 1998). Reproduces from seeds in the wild (Kitajima et al., 

2006). Answer: yes with negligible uncertainty. 

ES-11 Is the taxon self-compatible or apomictic? (y = 1, n = -1, or ? = 0) 

Definitions (from Holmes, 1979): 

 Allogamy – Cross-fertilization. 

 Apomixis – A reproductive process without fertilization in plants, (i.e., asexual 

reproduction through seeds). 

 Autogamy – Self-fertilization. 

 Chasmogamy – Opening of a mature flower in the normal way to ensure fertilization. 

 Cleistogamy – The condition of having flowers which never open and are self-pollinated, 

and are often small and inconspicuous. 

 Dioecious – Having male and female flowers on different plants.

 Geitonogamy – Fertilization of a flower by another from the same plant. 

 Monoecious – Having unisexual male and female flowers on the same plant. 

Rationale. Taxa that can produce seed without out-crossing can spread from seed produced by 

isolated plants. 

Criteria. Answer yes if a single individual of the taxon is capable of sexual reproduction, even if 

the self-fertilization produces fewer viable seeds than cross-pollination does. Self-compatible 

(including cleistogamous) or apomictic taxa would receive a yes answer to this question. 

Dioecious taxa should receive a no answer, unless there is specific evidence they are capable of 

apomixy. Direct evidence on whether or not the taxon is self-compatible is necessary; answer 

unknown where direct evidence is lacking. 

Notes 

 Search taxon name + “self*,” “crossing,” “apomi*,” “reproduction,” “fertile,” “fertility,” 

“geiton*,” “cleistogam*,” “dioecious,” “allogamy,” “autogam.” 

 The horticultural literature may refer to plants as “self-seeding.” This generally means 

that plants produce seeds that drop and germinate in subsequent years (i.e., self-sow). 

Describing plants as “self-seeding” or “self-sowing” does not address how the seeds (and 

their embryos) were produced, and therefore does not necessarily mean they are selfcompatible. A self-incompatible plant may have gotten pollen from another plant and 

then “self-seeded.” 
TP E-300, Ver. 2.3 February 11, 2019 74

Full examples 

 Nassella neesiana is autogamous and exhibits a selfing rate of close to 100 percent (Vidal 

et al., 2011). It produces cleistogenes (seeds produced through cleistogamy) in flowering 

stem joints where they are often concealed by leaf sheaths (Richardson et al., 2006). 

Answer: yes with negligible uncertainty. 

References 

 Bicknell, R. A., and A. M. Koltunow. 2004. Understanding apomixis: Recent advances 

and remaining conundrums. The Plant Cell Online 16(suppl 1):S228-S245. 

 Holmes, S. 1979. Henderson's Dictionary of Biological Terms (9th ed.). Van Nostrand 

Reinhold Company, New York. 510 pp. 

 Schön, I., K. Martens, and P. van Dijk (eds.). 2009. Lost Sex: The Evolutionary Biology 

of Parthenogenesis. Springer, Dordrecht. 615 pp. 

ES-12 Does the taxon require specialist pollinators that are not present in the WRA area? 

(y = -1, n = 0, or ? = 0) 

Rationale. Specialist pollinators are organisms that pollinate only a limited number of plant taxa 

(Johnson and Steiner, 2000) and have usually coevolved with the plants they pollinate (e.g., 

Ficus plants and fig wasps). Plant taxa that depend on specialist pollinators for sexual 

reproduction are less likely to become invasive when introduced to a new area without their 

pollinators. 

Criteria. Answer yes if the taxon is obligately dependent on a particular pollinator taxon for 

sexual reproduction, and that pollinator is not present in the area of introduction. Plant taxa that 

are likely to be pollinated by a different pollinator or set of pollinators in the WRA area should 

not receive a yes, even if such pollination may be uncommon. In these cases, document your 

reasoning and evidence and describe your uncertainty. Answer no for taxa that require specialist 

pollinators that are present in the WRA area, for taxa with documented generalist pollinators, or 

for taxa that reproduce without pollinator assistance (e.g., taxa that are wind pollinated, 

cleistogamous, etc.). Because the great majority of grasses (Poaceae), sedges (Cyperaceae), and 

rushes (Juncaceae) are wind-pollinated (Faegri and Van der Pijl, 1979), assume no with low 

uncertainty for these taxa if direct evidence is lacking. Also assume no with low uncertainty for 

all ferns and fern allies. Otherwise, answer unknown. 

Notes 

 Search taxon name + “pollinat*.” 

 Although pollinators can be categorized as specialists or generalists, pollinators cover the 

entire spectrum between those extremes (Johnson and Steiner, 2000), and it may be 

difficult to classify them either way. In these cases, describe your evidence and reasoning 

and increase uncertainty as necessary. 

 Floral visitors are not the same as pollinators. Many insects visit flowers or take nectar 

and pollen resources that plants provide without actually transferring pollen to the stigma 

of the flowers. Use higher levels of uncertainty when it is not clear that a floral visitor is 

an effective pollinator. 
TP E-300, Ver. 2.3 February 11, 2019 75

Full examples 

 In its native habit in South Africa, Gladiolus undulatus is pollinated by long-proboscid 

flies with specialized elongated mouthparts (Goldblatt and Manning, 1999). Gladiolus 

undulatus does not set seed in Australia where the long-proboscid flies are not present 

(Western Australian Herbarium, 2011). Answer: yes with low uncertainty. 

 Penstemon barbatus is primarily adapted for hummingbird pollination. In a study by 

Lange et al. (2000), large reductions in seed set resulted from hummingbird exclusion. 

Two of the hummingbird species known to pollinate it (rufous and calliope) are present in 

Canada, but found only in the west (NAPPC, 2007). It is not known if Canada's most 

wide-ranging hummingbird (ruby-throated) would pollinate this species. Some areas of 

Canada have no hummingbirds. To a much lesser extent, P. barbatus is pollinated by 

insects (e.g., halictid bees) (Lange et al., 2000), which are present in Canada. We 

answered no because hummingbirds and halictid bees represent two large guilds of 

pollinators and are not “specialized enough” for the scope of this question. However, 

because hummingbirds are not widely distributed in Canada and because halictid bees are 

not as effective as hummingbirds, we used high uncertainty. Answer: no with high 

uncertainty. 

References 

 Johnson, S. D., and K. E. Steiner. 2000. Generalization versus specialization in plant 

pollination systems. Trends in Ecology and Evolution 15(4):140-143. 

 Faegri, K., and L. Van der Pijl. 1979. The Principles of Pollination Ecology. Pergamon 

Press, Oxford. 244 pp. 

ES-13 What is the taxon’s minimum generation time? Choose one: (a) less than a year with 

multiple generations per year; (b) 1 year, usually annuals; (c) 2 or 3 years; (d) more than 3 

years; or (?) Unknown. (a = 2, b = 1, c = 0, d = -1, ? = 0). 

Rationale. Generation time is the length of time between one plant generation and the next at the 

same life-cycle stage or point in the life cycle (e.g., germinating seed to germinating seed, corm 

to corm, vegetative offshoot to an offshoot of the same size; sensu Ricklefs and Miller, 2000). 

This applies to both sexually and vegetatively reproducing plants. For vegetatively produced 

plants consider generation time as the time taken for a plant to produce a clone (e.g., ramet) that 

is capable of independent growth. However, some plants remain attached to their parents for 

some time. 

Criteria. Use specific generation time data on the taxon, to answer this question, unless a taxon’s 

life form (e.g., a woody tree) can help eliminate one or more answer choices. Otherwise, answer 

unknown. 

 Plants typically completing their life cycle in a year (i.e., annuals) should usually get an 

answer of “b.” Many plants are annuals that germinate, grow, and reproduce in a few 

months, but produce seeds that remain dormant for the rest of the year; answer “b” for 

these plants. For example, an annual may germinate in March and produce seed by the 

end of May, but seed dormancy is a part of a plant’s life cycle and the resulting progeny 

will not germinate until next spring. Therefore, concluding that the generation time is less 
TP E-300, Ver. 2.3 February 11, 2019 76

than a year would be incorrect. Many seeds are dormant at maturity due to a variety of 

morphological and physiological mechanisms (Baskin and Baskin, 2004). 

 Answer “a” for taxa that produce more than one generation per year under natural 

conditions, i.e., multiple generations per year are possible. This may be uncommon for 

plants reproducing by seed but some vegetatively reproducing plants can produce 

multiple generations in a year. 

 Answer “c” for taxa that require two or three years to complete their generation time. In 

other words, these taxa will not produce a seed or vegetative propagules until their 

second or third year. Most biennials should receive a “c” response. If the answer is based 

on vegetative reproduction, then the life cycle starts and ends with vegetative propagules 

of the same size. 

 Answer “d” for taxa that require four or more years to complete their generation time. 

Most shrubs and trees will receive a “d.” 

You will not often find enough information to definitively answer this question. However, based 

on a taxon’s life form, you should be able to easily exclude one or two answer choices every 

time. For example, a tree that does not reproduce vegetatively is not going to have a minimum 

generation time of one year or less, and probably not even less than four years. Thus, an answer 

of “d,” with an alternate choice of “c” and moderate uncertainty may be the best answer. We 

recommended you select the best answer, alternate choices, and uncertainty level commensurate 

with the evidence available. 

Notes 

 Search taxon name + “years” + “flower” + “age.” 

 Information from horticultural or forestry sources may be helpful. 

 Do not assume that just because a plant is a biennial or perennial that it cannot reproduce 

in its first year. 

 Because ferns experience an alternation of generation where the sporophyte is the 

noticeable generation and the independent gametophyte is much more inconspicuous, 

consider the minimum generation time as the time to go from reproductive sporophyte to 

reproductive sporophyte. 

 For terrestrial vines that root along their nodes, determining generation time is very 

difficult because often each rooted segment is capable of independent growth. 

Furthermore, such taxa could simply continue to grow indefinitely. In these cases, it may 

be better to rely on information about sexual reproduction. If the vine taxa do not 

reproduce sexually, then consider how many years may go by before older portions of the 

vine die, leaving several independent segments. Whatever evidence and approach you 

use, clearly describe it in the notes column. 

Full examples 

 We found no information about this species’ generation time. Limnobium laevigatum is 

ecologically similar to water hyacinth, only smaller. Both are free-floating, herbaceous 

aquatics, have similar morphology, and reproduce vegetatively. Water hyacinth 

(Eichhornia crassipes) can double in population size in five days due to vegetative 

reproduction; furthermore, plants can begin blooming at three to four weeks of age 

(Parsons and Cuthbertson, 2001). We are conservatively answering “b” for L. laevigatum, 
TP E-300, Ver. 2.3 February 11, 2019 77

as it seems very likely that new offshoots will be produced within a year. In fact, we 

suspect that there may be several generations of vegetative offshoots in a year, potentially 

supporting an “a” response. Using high uncertainty because of a lack of information for 

this species and because we had to rely on information for a similar species. Alternate 

answers for the stochastic simulation were “a” and “c.” Answer: “b” with high 

uncertainty. 

References 

 Baskin, J. M., and C. C. Baskin. 2004. A classification system for seed dormancy. Seed 

Science Research 14(1):1-16. 

 Ricklefs, R. E., and G. L. Miller. 2000. Ecology (4th ed.). W. H. Freeman and Company, 

New York. 822 pp. 

ES-14 Is the taxon a prolific seed producer? (y = 1, n = -1, or ? = 0) 

Criteria. The level of seed production must be met under natural conditions and applies only to 

viable seeds. For herbaceous taxa, this rate should be >5,000/m2 crown area per year to receive a 

yes answer; for woody taxa, a rate of >1,000/m2

 crown area per year would receive a yes answer. 

Assume yes for ferns unless contradictory evidence exists. For this question, specific evidence is 

needed to support both yes and no answers. Ideally, use quantitative information when available. 

If specific data on this attribute are unavailable, extrapolate seed production from data on 

numbers of seeds per flower or fruit, and number of flowers or fruit per individual of average 

size, per unit area. Qualitative statements may be used to support no answers but should only be 

used to answer yes if it is confirmed by several references and consistent with other aspects of 

the species’ biology. If you cannot find sufficient information, answer unknown. 

Notes 

 Search taxon name + “seed.” 

 Botanical sketches or pictures of the plant, flowers, and/or fruit can be used to estimate 

the seed/fruit output, but increase uncertainty accordingly. 

 If answers are based on qualitative statements, use higher levels of uncertainty 

Full examples 

 The maximum productivity of Falcaria vulgaris (an herbaceous species) is up to 900 

achenes per plant (AgroAtlas, 2012), but this reference does not indicate the number of 

plants per square meter. Note that achenes are fruit that contain only one seed each. 

Falcaria vulgaris produces several thousand seeds (Korman, 2011). Germination rates of 

70-90 percent in the lab and 24-54 percent in the greenhouse are reported (Korman, 

2011). Based on stem densities of 24 to 40, to possibly 645 per square meter (Korman, 

2011), production of hundreds of achenes, and pictures of plant populations in South 

Dakota, it very likely this herbaceous species produces more than 5000 seeds per square 

meter. A researcher and manager who has studied Falcaria believes it produces more 

than 5000 seeds per square meter (Korman, 2012). Answer: yes with low uncertainty. 
TP E-300, Ver. 2.3 February 11, 2019 78

ES-15 Are propagules likely to be dispersed unintentionally by human activity? (y = 1, n = -

1, or ? = 0) 

Criteria. Answer yes if human activity has resulted or is likely to result in unintentional dispersal 

of propagules (any structure, sexual or asexual, which serves as a means of reproduction) of the 

taxon. Taxa found in disturbed habitats or habitats frequented by people, and having 

morphological dispersal adaptations such as burrs, sticky seeds, or small seeds that might be 

carried on vehicle tires or on shoes should receive a yes answer. Also consider propagules 

dispersed in yard waste, soil, or other debris, and as contaminants of machinery or other human 

dispersal vectors (see below for more examples). Answer no if you find no evidence of previous 

dispersal through human activity and absence of adaptations for such dispersal. If no information 

exists, answer unknown. 

Notes 

 Search taxon name + “seed dispersal.” 

 Greater weight should be placed on evidence of unintentional dispersal by humans than 

on seed size alone. 

 The next question (ES-16) focuses on propagules dispersing on or in agricultural, 

forestry, or horticultural products. Do not count that evidence here if it better applies 

there. Some examples could qualify as evidence under either ES-15 or ES-16. In these 

difficult cases, use the evidence where most appropriate, but in only one answer. 

 Some examples of human dispersal vectors include the following: boats, vehicles, trains, 

hiking and other outdoor equipment, agricultural machinery, personal equipment, and 

yard waste. 

Full examples 

 Dittrichia graveolens is spreading along roadways in California from the San Francisco 

area into the Central Valley (Hrusa et al., 2002). Seeds attach to clothing and bags, and 

move in sand and gravel for road construction (Parsons and Cuthbertson, 2001). 

Contaminates vehicles and equipment (Moerkerk, 2006), and likely spreading that way in 

California (Brownsey et al., 2013). Answer: yes with negligible uncertainty 

ES-16 Are propagules likely to be dispersed as contaminants or hitchhikers of any 

agricultural, forestry, or horticultural products? (y = 2, n = -1, or ? = 0) 

Rationale. In this question agricultural, forestry, or horticultural products are any of the raw or 

unprocessed, or partially processed plant or animal products of these industries that may be 

capable of carrying viable plant propagules (see examples below). 

Criteria. A lack of positive evidence for this question and a growth form, biology, ecology, or 

habitat that makes contamination unlikely results in a no answer, since contaminants and 

hitchhikers are likely to be reported in the literature or by quarantine authorities. Otherwise, 

answer unknown. 

Notes 

 Search taxon name + “contaminant,” “pathway.” 
TP E-300, Ver. 2.3 February 11, 2019 79

 Search CAB Abstracts and Agricola, or any other relevant databases. Search national pest 

interception databases if available. 

 Examples of contaminants include: 

1. Seeds of taxa included as contaminants in shipments of grain or imported seeds or 

fruit 

2. Propagules contaminating hay, wood, mulch, etc. 

3. Seeds likely to be included in potpourri mixtures 

4. Seeds with cut flowers or decorative branches 

5. Seeds stuck or caught on shipping containers, ventilation grills, pallets, or other 

transportation conveyances. 

 References to “potential seed contaminant” in lists such as the GRIN database are 

insufficient without direct evidence. However, such statements can be used to support 

higher uncertainty levels. 

Full examples 

 Unknown. Vegetative fragments of Nymphoides cristata could be present in materials 

distributed within the aquarium trade (Maki and Galatowitsch, 2004). Answer: unknown 

with maximum uncertainty. 

 Nassella neesiana has barbed seeds. It was introduced to the Waipawa area of New

Zealand in contaminated pasture seed (Slay et al., 1999). It moves in hay bales (Weller et 

al., 2012). Because it is similar in appearance to desirable hay species in the vegetative 

state, it may be overlooked by managers (Weller et al., 2012). It is present in Genoa, 

Italy, near leather tanning facilities that processed hides from Argentina (Haywood and 

Druce, 1919). Several taxa of Nassella are wool adventives in Europe (Verloove, 2005). 

Answer: yes with negligible uncertainty. 

ES-17 What is the number of natural dispersal vectors as described by ES-17a through ES17e? (none = -4, one = -2, two = 0, three = 2, four or five = 4) 

Criteria. This question is automatically answered by the spreadsheet based on the answers to 

questions ES-17a through ES-17e. The answer should be a number from 0 to 5. A single 

uncertainty value for the five sub-questions is not produced; instead, the five uncertainties are 

directly incorporated into the index of uncertainty for this risk element. For this row in the 

spreadsheet, enter a general description of the properties of the fruit, seeds, and/or other 

propagules in the notes column that will help support determinations for the five sub-questions. 

Notes 

 You can use congeneric information to answer questions ES-17a through ES-17e if the 

propagule traits of the congener are similar to the taxon being assessed. In these cases, 

use a higher level of uncertainty and note that the answer is based on congeneric 

information. 

References 

 van der Pijl, L. 1982. Principles of Dispersal in Higher Plants (3 ed.). Springer-Verlag, 

Berlin. 214 pp. 
TP E-300, Ver. 2.3 February 11, 2019 80

Full examples 

 Fruit and seed description for questions ES-17a through ES-17e: Cordia curassavica has 

red-colored, fleshy fruit with thin testas (Opler et al., 1975). Seeds are 4.5-5 x 3.5 mm 

(Reed, 1977). Red-colored fruit (5 mm across) have one seed each (Swarbrick, 1997). 

ES-17a Are propagules wind dispersed? (y, n, ?) 

Criteria. Answer yes if evidence demonstrates that wind contributes significantly to the dispersal 

of propagules. Even without such documentation, taxa with morphological features that facilitate 

propagule movement by wind (achenes with a pappus, samaras, etc.) should receive a yes 

response. This group includes tumbling plants and fern spores. If fruit or seed do not have traits 

indicating wind dispersal, answer no, except answer unknown when the evidence is ambiguous 

or there is no information leading one answer or the other. Some references may claim a species 

is wind dispersed when seeds possess no obvious adaptation for wind dispersal, or when a breeze 

just helps knock seeds off swaying parent plants. Answer no or unknown in these cases, as 

appropriate, and use higher uncertainty. 

Notes 

 Search taxon name + “seed dispers*,” “wind.” 

 Online databases that include information on the means of dispersal include BIOLFLOR 

for Germany (www.biolflor.de), and the Kew Gardens database 

(http://data.kew.org/sid/dispersal.html), but the source literature should also be checked. 

Full examples 

 Hakea sericea seeds are winged (Gordon, 1993b; Gunn and Ritchie, 1988; Weber, 2003). 

Wind disperses seeds over several kilometers (Richardson et al., 1997). Seeds are 

dispersed long distances by wind, forming the nucleus of new invasions (CABI, 2012). 

Seeds are dispersed well by wind (Richardson et al., 1987). Hakea are wind-dispersed 

(Groom, 2010). Answer: yes with negligible uncertainty. 

References 

 Sheldon, J. C., and F. M. Burrows. 1973. Dispersal effectiveness of achene-pappus units 

of selected Compositae in steady winds with convection. New Phytologist 72(3):665-675. 

ES-17b Are propagules water dispersed? (y, n, ?) 

Criteria. Evidence that the propagule is carried by and survives in water, or is buoyant, results in 

a yes response. Answer no with low or negligible uncertainty where there is evidence that 

propagules are not buoyant or dispersed by water. If fruit or seed characters or other 

biogeographical evidence suggest that the taxon is not dispersed by water, answer no. Otherwise, 

answer unknown. Just about any seed or other dispersal propagules can be dispersed by water if 

the water flow is strong enough (e.g., floods, tidal surges, etc.), and some sources may claim a 

taxon is water dispersed based on these rare events. In these circumstances, answer no but with 

higher uncertainty. Only answer yes if the taxon routinely disperses by water under normal (i.e., 

average) environmental conditions. If multiple, independent sources claim a taxon is dispersed 

by water when there is no other type of supporting evidence available, answer yes with higher 
TP E-300, Ver. 2.3 February 11, 2019 81

uncertainty as appropriate. In difficult cases, consider where the species naturally occurs and 

other aspects of its biology. For example, aquatic and wetland taxa are likely to be dispersed by 

water. Species that grow only along streams, rivers, or other water bodies may also be routinely 

dispersed by water. 

Notes 

 Search taxon name + “buoyant,” “floats” + “water dispers*.” 

 See online databases listed under ES-17a. 

 Examples of fruit or seed adaptations that may facilitate water dispersal include 

unwettable seed coats, seeds or fruit that contain air spaces, cork or oil, and hairs or slime 

on the seed coat, which provide resistance to sinking (Howe and Smallwood, 1982). 

 Documented distribution along waterways and other relevant ecological information 

provides supporting evidence for a yes response, but use higher uncertainty without other 

supporting evidence. 

Full example 

 The propagules of Colophospermum mopane are dispersed by water (Melusi and 

Mojeremane, 2012; Sharma et al., 1989). In a field study, the authors concluded that “the 

most likely agents of dispersal of mopane diaspores to be water and wind,” and flotation 

was an important means of dispersal (Styles and Skinner, 1997). Furthermore, “[a] 

characteristic of the summer rainfall period is the subsequent sheetflow of water along 

the surface of the ground”; this run-off collects masses of mopane diaspores and deposits 

them in piles along the drainage lines. We answered yes with low uncertainty because 

multiple references mention dispersal by water, it has broad pods that could likely float 

easily, and flooding occurs yearly in its native habitat. Answer: yes with low uncertainty. 

References 

 Howe, H. F. and J. Smallwood. 1982. Ecology of Seed Dispersal. Annual Review of 

Ecology and Systematics 13: 201-228. 

ES-17c Are propagules dispersed by birds? (y, n, ?) 

Criteria. Dispersal of propagules by birds includes transport in a viable form externally on birds 

(on feet or feathers) or through their digestive tracts (excluding flightless birds). Evidence of bird 

dispersal is sufficient for a yes response. Where you find no information, assume yes for fleshy 

fruits that are less than 3–4 cm in diameter. Taxa known not to be dispersed by birds, that have 

seeds/spores known to always be digested, or with propagules too large for such dispersal should 

receive a no answer. Otherwise, answer unknown. 

Notes 

 Search taxon name + “seed dispers*,” “birds.” 

 See online databases listed under ES-17a. 

 Taxa with small red fruits and indigestible seeds, e.g., Schinus terebinthifolius (Panetta, 

1997) would receive yes answers. Taxa with cones that are opened by birds to release the 

seeds would also receive a yes response. 
TP E-300, Ver. 2.3 February 11, 2019 82

 When there is evidence of bird dispersal but no information about seed viability or the 

effectiveness of bird dispersal, answer yes, but use higher uncertainty. When an answer 

of yes is based on fleshy fruit morphology alone, use high uncertainty. 

Full examples 

 The fruits of Persicaria chinensis “...are berries, globose in shape. Enclosed in the 

enlarged and fleshy calyx at maturity...Seeds small, black, fruits edible, sour tasting” 

(Tabish and Girija, 2011). The seeds of Persicaria chinensis are dispersed by birds 

intentionally eating the seed (Royal Botanic Gardens Kew, 2008). “Its fruits are bird 

dispersed” (Goodland and Healey, 1996). Additionally, the related species, Persicaria 

perfoliata (mile-a-minute weed) has seed that is spread by birds (Swearingen et al., 

2010). Answer: yes with negligible uncertainty. 

References 

 Panetta, F. D., and J. McKee. 1997. Recruitment of the invasive ornamental, Schinus 

terebinthifolius, is dependent upon frugivores. Australian Journal of Ecology 22:432-438. 

ES-17d Are propagules dispersed by other animals (externally)? (y, n, ?) 

Criteria. This dispersal type includes seeds likely to be dispersed by (non-avian) animals, in 

hooves/feet or on fur. It also includes, some plant taxa that have seeds with an oily or fat-rich 

organ (an elaiosome) that aids in dispersal by ants. Taxa that dispersed by animals in this fashion 

will have adaptations (e.g., burrs), and/or grow in situations that make it likely that propagules 

become temporarily attached to the animal (non-avian). When a propagule clearly has 

adaptations for external dispersal, but you find no evidence that it is dispersed by animals 

externally, answer yes, but use high uncertainty. Where direct evidence is lacking, assume no for 

propagules that clearly have no mechanisms of attachment. 

Notes 

 Search taxon name + “seed dispers*.” 

 See online databases listed under ES-17a. 

Full examples 

 We found no evidence, and the species is well studied. Answer: no, with low uncertainty. 

 The seeds of Nassella neesiana readily bore into the skins of animals (Haywood and 

Druce, 1919), and seeds often disperse on wool, hides, and animal carcasses (Bourdôt et 

al., 2012). The backward-pointing hairs on the apex of the seed help anchor seeds on 

animal fur (Storrie and Lowien, 2003). Seeds can fall from fleece several months later; 

after five months, unshorn sheep still had 10 percent of seeds remaining (Gardener et al., 

2003a). Answer: yes with negligible uncertainty. 

ES-17e Are propagules dispersed by other animals (internally)? (y, n, ?) 

Criteria. Answer yes if propagules of the taxon pass through the digestive tract of animals 

(including bats and flightless birds but excluding flying birds) and maintain viability. If fruit are 
TP E-300, Ver. 2.3 February 11, 2019 83

consumed (along with seeds) but you find no evidence that seeds pass remain intact and viable, 

answer yes with higher uncertainty if the animals are known to effectively disperse seeds of 

other, similar species. Answer no if the taxon is unlikely to be eaten by animals or if seeds do not 

remain viable after passage through the gut. Answer unknown if neither data nor morphological 

traits clarify whether this means of dispersal is likely. 

Notes 

 Search taxon name + “seed dispers*,” “animal,” “gut,” “droppings.” 

 See online databases listed under ES-17a. 

Full examples 

 Fruits of Vitex rotundifolia are eaten by birds and squirrels, which may disperse the seeds

(Suiter, 2005). “Could be dispersed by animals but I do not think they care for the odor or 

other chemicals. The primary dispersal is by water or people” (Whitwell, 2010). A 

naturalist has not seen deer or any other animal (other than birds) eat the fruit (Dewire, 

2010). Collared peccaries consume and disperse the seed of Vitex mollis (MartinezRomero and Mandujano, 1995). Answer: unknown with maximum uncertainty. 

 Phyllanthus maderaspatensis seeds were able to germinate after being found in tortoise 

feces (Hnatiuk, 1978). Answer: yes with low uncertainty. 

References 

 Willson, M. F. 1993. Mammals as seed-dispersal mutualists in North America. Oikos 

67(1):159-176. 

ES-18 Is there evidence that a persistent (>1 year) seed bank (or other propagules) is 

formed? (y = 1, n = -1, or ? = 0) 

Rationale. Long-term seed viability increases the invasive potential of a taxon by enabling them 

to escape unfavorable conditions in time rather than space. 

Criteria. For a yes response, greater than 1 percent of the seeds in the soil should remain viable 

for more than 12 months. Consider both soil and canopy seed banks. If seeds of the taxon require 

scarification or other dormancy-breaking treatments for germination, answer yes with high 

uncertainty unless that treatment naturally and uniformly occurs within 12 months of seed 

production. Documented lack of any type of seed dormancy (including dormancy induced and/or 

enforced by burial) supports a no answer. Otherwise, answer unknown. 

Although this question generally applies to seeds, in some cases plants may form other types of 

persistent propagules, such as rhizomes, that can escape unfavorable conditions (beyond yearly 

seasonal changes). For example, Epipactis helloborine, a temperate terrestrial orchid, forms 

rhizomes that can remain dormant for 7-18 years, thereby escaping drought conditions (Light and 

MacConaill, 2004). If you find such evidence, then answer yes, but only for extreme examples. 

We do not intend this question to result in yes answers for the type of seasonal “dormancy” 

found in many plant structures (e.g., bulbs, corms, rhizomes, etc.). 
TP E-300, Ver. 2.3 February 11, 2019 84

Notes 

 Search taxon name + “seed storage,” “seed bank,” “recalcitrant,” “seed” + “germination,” 

“viability” + “years,” “seed” + “dorman*.” 

 Data on seed viability for more than 10,000 taxa exist at the Kew Gardens Seed 

Information Database (http://data.kew.org/sid/), but the source literature should be 

checked. 

 Do not use data on seed viability from controlled laboratory storage conditions to answer 

this question. For example, seed stored in liquid nitrogen can remain viable indefinitely 

and seeds stored under cool, dry, and dark conditions can remain dormant far longer than 

under field conditions. 

 Thompson et al. (1997) details seed banks in northern temperate floras. Baskin and 

Baskin’s (1998) book on seeds includes information on dormancy and seed banks. 

 Sometimes a source may refer to a soil seed bank but is simply discussing seeds in the 

soil irrespective of how long they have been or will be there. Remember that a yes 

response requires evidence of persistence for more than a year. 

Full examples 

 Freshly matured seeds of Geranium lucidum have water impermeable seed coats (Van 

Assche and Vandelook, 2006). Seed burial experiments showed that seeds remain viable 

for more than one year and need a period of desiccation to break dormancy (Van Assche 

and Vandelook, 2006). Experience from managers controlling populations suggests it has 

a long-term seed bank (Taylor, 2006). Answer: yes with negligible uncertainty. 

References 

 Baskin, C. C., and J. M. Baskin. 1998. Seeds, Ecology, Biogeography and Evolution of 

Dormancy and Germination. Academic Press, San Diego, CA, U.S.A. 666 pp. 

 Light, M. H. S., and M. MacConaill. 2004. Appearance and disappearance of a weedy 

orchid, Epipactis helleborine. Pages 77-93 in International Orchid Workshop. Acad Sci 

Czech Republic Inst Botany, Haapsalu, ESTONIA. 

 Levin, D. A. 1990. The seed bank as a source of genetic novelty in plants. The American 

Naturalist 135(4):563-572. 

 Thompson, K., J. P. Bakker, and R. M. Bekker. 1997. The Soil Seed Bank of NorthWest 

Europe: Methodology, Density and Longevity. Cambridge University Press, Cambridge. 

276 pp. 

ES-19 Does the taxon tolerate or benefit from mutilation, cultivation, or fire? (y = 1, n = -1, 

or ? = 0) 

Rationale. Mutilation, cultivation, and fire are artificial treatments or natural disturbances that 

can remove or kill most of the biomass of a plant. Taxa that tolerate or benefit from such 

disturbances may out-compete other taxa. 

Criteria. Answer yes if the taxon tolerates or benefits from any physical biomass removal, 

including those intended for control purposes. Taxa adapted to fire or grazing pressure are likely 

to have yes responses. Taxa that over-compensate to tissue loss by herbivory should also receive 
TP E-300, Ver. 2.3 February 11, 2019 85

a yes answer. Use explicit evidence of response/non-response to biomass loss for yes or no 

answers. Answer unknown if direct evidence is lacking. 

Notes 

 Search taxon name + “coppice,” “resprout.” Many species are capable of resprouting 

after being cut. In these cases, look for evidence that the species resprouts more 

vigorously than most other species, or that resprouts occur along the root system, not just 

from the base of the stem/trunk. 

 This question does not apply to seed banks or how seeds respond to disturbance. 

Full examples 

 Falcaria vulgaris produces a deep taproot (at least 35 cm; Gress, 1923). Experiments 

where the taproot was cut into 4 cm long portions and buried at 5 cm soil depth showed 

that the root pieces had high shoot regeneration rates (62-82 percent) (Korman, 2011). 

This species has survived near a greenhouse at South Dakota State University since the 

1970s despite repeated mowing (Korman, 2011). In South Dakota, plant densities are 

higher in areas with pocket gopher tunnels, possibly due to their foraging activity 

breaking roots and creating new vegetatively produced plants (Korman, 2011). Answer: 

yes with low uncertainty. 

ES-20. Does the taxon have resistance to some herbicides or demonstrate potential to 

acquire herbicide resistance? (y = 1, n = 0, or ? = 0) 

Definitions [from WSSA, 1998] 

 Herbicide resistance: “The inherited ability of a plant to survive and reproduce following 

exposure to a dose of herbicide normally lethal to the wild type. In a plant, resistance may 

be naturally occurring or induced by such techniques as genetic engineering or selection 

of variants produced by tissue culture or mutagenesis.” 

 Herbicide tolerance: “The inherent ability of a species to survive and reproduce after 

herbicide treatment. This implies that there was no selection or genetic manipulation to 

make the plant tolerant; it is naturally tolerant.” 

Rationale. Herbicide resistance is likely to promote invasiveness or weediness of taxa in 

environments subject to frequent herbicide applications. 

Criteria. Very few plants will score a yes as less than 250 species are officially recognized to be 

resistant to herbicides (Heap, 2013). Determine if the taxon is one of the few plants to have 

developed herbicide resistance, but be careful to distinguish between herbicide tolerance and 

herbicide resistance (see the official definitions from the Weed Science Society of America 

above). If the taxon has resistance, answer yes and use low or negligible uncertainty as 

appropriate. If the taxon has a congener or other close relative with herbicide resistance, then it 

may be able to acquire this trait through gene flow, or it may carry the gene for the underlying 

trait but not yet have undergone selection. In such cases, answer yes if you find supporting 

evidence for interspecific gene flow or if the plant occurs in places where it is likely to be 

regularly subjected to herbicide applications, which would provide a selective force for herbicide 

resistance. If you find no positive evidence, then answer no. Otherwise, answer unknown. 
TP E-300, Ver. 2.3 February 11, 2019 86

Notes 

 http://www.weedscience.com has a list of plants with known herbicide resistance. 

 Search taxon genus + “herbicide” + “resist*.” 

 If the answer is based on congeneric information, use higher levels of uncertainty 

(moderate or high). 

Full examples 

 We found no evidence that Xanthoceras sorbifolium is resistant to herbicides. 

Furthermore, it is not listed by Heap (2013). Given that this species is not a weed and 

does not occur in environments where herbicides are heavily used (e.g., row crops), it is 

unlikely to have developed herbicide resistance. Answer: no with low uncertainty. 

References 

 Heap, I. 2013. The International Survey of Herbicide Resistant Weeds. Online. Available: 

http://www.weedscience.com/summary/home.aspx. 

 WSSA. 1998. Technology Notes. Weed Technology 12(4):789-790. 

ES-21 Adaptive potential: Number of plant hardiness zones (out of 13) suitable for 

survival. (zero to three = -1, four to nine = 0, ten to thirteen = 1) 

Rationale. The latitudinal range of a species is associated with invasiveness (Goodwin et al., 

1999; Reichard, 2001). Species with broad latitudinal ranges are more likely to be invasive. 

Although many abiotic factors change with latitude, temperature is one of the most important. 

This question evaluates one possible measure of adaptive potential by measuring the number of 

Plant Hardiness Zones suitable for the species. The PPQ WRA was validated with the global 

Plant Hardiness Zones established by Magarey et al. (2008). 

Criteria. This question is automatically answered in the spreadsheet based on the answers to 

questions Geo Z1-Z13. The answer should be a number from 1 to 13. An uncertainty value for 

this question is not generated. 

References 

 Goodwin, B. J., A. J. McAllister, and L. Fahrig. 1999. Predicting invasiveness of plant 

species based on biological information. Conservation Biology 13(2):422-426. 

 Magarey, R. D., D. M. Borchert, and J. Schlegel. 2008. Global plant hardiness zones for 

phytosanitary risk analysis. Scientia Agricola 65 (Special Issue):54-59. 

 Reichard, S. 2001. The search for patterns that enable prediction of invasion. Pages 10-19 

in R. H. Groves, F. D. Panetta, and J. G. Virtue, (eds.). Weed Risk Assessment. CSIRO, 

Collingwood, Australia. 
TP E-300, Ver. 2.3 February 11, 2019 87

ES-22 Adaptive potential: Number of Köppen-Geiger climate classes (out of 12) suitable for 

survival. (zero to two = -2, three = 0, four to twelve = 2) 

Rationale. This question evaluates another measure of adaptive potential by measuring the 

number of different climate types suitable for the species. The PPQ WRA model was validated 

with the Köppen-Geiger system presented by Peel et al. (2007). 

Criteria. This question is automatically answered in the spreadsheet based on the answers to 

questions Geo-C1 through Geo-C12. The answer should be a number from 1 to 12. An 

uncertainty value for this question is not generated. 

References 

 Peel, M. C., B. L. Finlayson, and T. A. McMahon. 2007. Updated world map of the 

Köppen-Geiger climate classification. Hydrology and Earth Systems Sciences 11:1633-

1644. 

ES-23 Adaptive potential: Number of annual precipitation bands (out of 11) suitable for 

survival. (zero to four = -1, five to seven = 0, eight to eleven = 1) 

Rationale. This question evaluates another measure of adaptive potential by measuring the range 

of precipitation suitable for species survival. Species adapted to a wide range of conditions are 

expected to establish and spread more readily than those with narrow niches. The PPQ WRA 

model was validated with 11 ten-inch global precipitation bands generated from climate data and 

the procedure followed in Magarey et al. (2008). 

Criteria. This question is automatically answered in the spreadsheet based on the answers to 

questions Geo-R1 through Geo-R11. The answer should be a number from 1 to 11. An 

uncertainty value for this question is not generated. 

References 



 Magarey, R. D., D. M. Borchert, and J. Schlegel. 2008. Global plant hardiness zones for 

phytosanitary risk analysis. Scientia Agricola 65 (Special Issue):54-59. 
TP E-300, Ver. 2.3 February 11, 2019 88

Appendix F. Question-specific guidance and examples for the risk element 

Impact Potential 

This risk element categorizes a species’ capacity for direct and indirect impacts in production 

(P), natural (N), and anthropogenic (A) systems based on existing evidence. Production systems 

include, but are not limited to, croplands, container and field nurseries, orchards, pastures, 

rangelands, and forest plantations. Natural systems refer to wild or conservation lands that are set 

aside or managed for their natural biological diversity (e.g., national/state parks, preserves, 

conservation areas, wildlands, forested areas). Anthropogenic systems refer to human landscapes 

such as cities and suburbs or other non-agricultural areas frequently associated with or managed 

and disturbed by people (e.g., gardens, lawns, recreational areas, roadways, railroad corridors, 

construction sites). Two general impacts (G) are also assessed. Because terms used in the 

invasive plant literature confound spread and impact, pay careful attention to how sources use 

this terminology when you answer the questions. 

For each question in the guidelines below, we provide the full question, answer choices, and 

scoring in bold type. Following that is the question-specific guidance. When necessary, 

additional suggestions, definitions, search criteria, and references are provided in bullets. For 

each question, one or more examples for weed risk assessment are provided. 

Imp-G1 Is the taxon allelopathic? (y = 0.1, n = 0, or ? = 0) 

Rationale. Plants that suppress the growth of other species through chemical means are 

allelopathic. Evidence of allelopathy is rare throughout the plant kingdom. 

Criteria. Often the only available evidence is from laboratory studies that used artificial 

concentrations of plant tissue extracts on a few test species. For the purpose of this question, we 

consider such evidence to be insufficient to merit a yes as this evidence is likely based on 

unnatural concentrations and artificial conditions. Answer yes if the evidence is derived from 

field studies or if the experimental evidence involved the use of non-concentrated plant tissues. 

Although rare, field-derived studies usually involve planting an indicator species in association 

with the plant believed to be allelopathic, or in soil obtained from the allelopathic plant, and then 

comparing to a control. Where data rely on concentrated extracts or little is known about the 

taxon being evaluated, answer unknown. Answer no with negligible uncertainty if the taxon has 

been documented not to be allelopathic. A lack of reported or suggested allelopathy for wellknown and studied taxa should generally result in a no response with low uncertainty. 

Notes 

 Search taxon name + “allelopath*.” Evidence is usually obtained from primary literature, 

which must be examined to evaluate the experimental evidence. 

 See Qasem and Foy (2001) for a review of allelopathic effects of multiple agricultural 

weeds. 

 See Radosevich et al. (2007, p. 239) for a list of some plants with allelopathic 

compounds. 

 Congeneric information may be used here, but use higher uncertainty. 
TP E-300, Ver. 2.3 February 11, 2019 89

Full examples 

 Several studies and data sheets have reported that Mikania micrantha produces 

allelopathic chemicals that inhibit the growth of other plants (ISSG, 2010; Tiwari et al., 

2005; Wu et al., 2010; Zhao and Peng, 2009), and that this has helped its invasion in 

China (Xie et al., 2010). Chen et al. (2009) examined allelopathy under field conditions 

and using soil from directly underneath M. micrantha plants. They showed that soil and 

plant extracts of M. micrantha inhibited radish seed germination, radicle length, and 

shoot length, and concluded that “allelopathic chemicals from M. micrantha not only 

affect other plants, but also soil nutrient properties.” Based on the anecdotal evidence 

from numerous other studies and the outcomes from the Chen et al. (2009) field study, 

we have little doubt this species is allelopathic. Answer: yes with negligible uncertainty. 

 We found no evidence of allelopathy for Sideritis montana. One study using plant 

extracts found evidence of allelopathy for the congener S. italic (Basile et al., 2011). We 

answered “no” because this evidence was derived from artificial extracts in a laboratory 

study, and it was based on a congener. Answer: no with moderate uncertainty. 

References 

 Qasem, J. R., and C. L. Foy. 2001. Weed allelopathy, its ecological impacts and future 

prospects: a review. Journal of Crop Production 4:43-120. 

 Radosevich, S. R., J. S. Holt, and C. M. Ghersa. 2007. Ecology of Weeds and Invasive 

Plants: Relationship to Agriculture and Natural Resource Management. Wiley, Hoboken, 

NJ., U.S.A. 454 pp. 

Imp-G2 Is the taxon parasitic, and does it have potential hosts in the WRA area? (y = 0.1, n 

= 0, or ? = 0) 

Rationale. A parasitic plant is a vascular plant that obtains nutrients and water from another plant 

through an attachment called a haustorium. Two main types of parasitic plants exist: stem 

parasites and root parasites (Nickrent et al. 2004). 

Criteria. Answer yes for any parasitic taxon with potential hosts present in the assessed area. 

This question includes holoparasitic (no chlorophyll) and hemiparasitic (chlorophyll present) 

plants. Parasitic plants are rare; a lack of positive evidence for this question results in a “no” 

answer. If your taxon is not in one of the plant families known to contain parasitic plants, then 

negligible uncertainty is appropriate. However, if your taxon is in one of these plant families, and 

you find no direct evidence of parasitism for your taxon, then use no with higher uncertainty or 

answer unknown, as appropriate. 

Based on information from the references below, the following families are known to contain 

parasitic plants: Amphorogynaceae, Apodanthaceae, Aptandraceae, Balanophoraceae, 

Cervantesiaceae, Comandraceae, Convolvulaceae, Coulaceae, Cynomoriaceae, Cytinaceae, 

Eremolepidaceae, Erythropalaceae, Hydnoraceae, Krameriaceae, Lauraceae, Lennoaceae, 

Loranthaceae, Misodendraceae, Mitrastemonaceae, Nanodeaceae, Octoknemaceae, Olacaceae, 

Opiliaceae, Orobanchaceae, Rafflesiaceae, Santalaceae, Schoepfiaceae, Scrophulariaceae, 

Strombosiaceae, Thesiaceae, Viscaceae, and Ximeniaceae. 
TP E-300, Ver. 2.3 February 11, 2019 90

Full examples 

 We found no evidence that Colophospermum mopane is parasitic. It does not belong to a 

family known to contain parasitic plants (Heide-Jorgensen, 2008; Nickrent, 2009; 

Walker, 2010). Answer: no with negligible uncertainty. 

References 

 Heide-Jorgensen, H. S. 2008. Parasitic Flowering Plants. Brill, Leiden, The Netherlands. 

438 pp. 

 Nickrent, D. L. and L. J. Musselman, L. J. 2004. Introduction to parasitic flowering 

plants. The Plant Health Instructor 13: 300-315. 

 Nickrent, D. 2009. Parasitic plant classification. Southern Illinois University Carbondale, 

Carbondale, IL, U.S.A. Last accessed June 12, 2009, 

http://www.parasiticplants.siu.edu/ListParasites.html. 

 Walker, R. 2014. Parasitic Plants Database. Rick Walker. 

http://www.omnisterra.com/bot/pp_home.cgi. 

Sub-element: Natural Systems 

This sub-element evaluates impacts the taxon may have in natural systems. Natural systems are 

areas (public or private) set aside for the conservation of nature and biotic diversity, including 

national or local parks, wildlife refuges, and habitat preserves. 

Imp-N1 Does the taxon change ecosystem processes and parameters? (y = 0.4, n = 0, or ? = 0) 

Rationale. This question focuses on species that significantly impact ecosystem processes and 

parameters that affect other organisms (e.g., changes fire regime, nutrient cycling, water 

availability). These kinds of species are sometimes called transformers. From Richardson et al. 

(2000): “Transformers are those taxa that have clear ecosystem impacts.” Several categories of 

transformers may be distinguished: (a) excessive users of resources (e.g., water, light, oxygen), 

(b) donors of limiting resources (e.g., nitrogen), (c) fire promoters/suppressors, (d) sand 

stabilizers, (e) erosion promoters, (f) colonizers of intertidal mudflats/sediment stabilizers, (g) 

litter accumulators, and (h) salt accumulators/redistributors. For examples of species causing 

each of these impacts, see Richardson et al. (2000). 

Criteria. A yes response to this question requires direct evidence of changes to ecosystem 

processes, or statements from more than one source that the taxon is changing or has changed 

ecosystem processes. Changes in community light levels must involve documented changes to 

one or more vegetation strata. If no evidence is available and the taxon is relatively well known 

or studied, then answer no; if the taxon is poorly known or you suspect it may be changing one 

of the listed processes, then answer unknown and provide your rationale. 

Full examples 

 Nymphoides peltata is a free-floating aquatic plant species whose growth decreases water 

oxygen levels, causing stagnant areas under the floating mats; additionally, it excludes 

light availability to the aquatic ecosystem and increases sedimentation (DCR, 2011; 

ISSG, 2012; Kelly and Maguire, 2009; van der Velde, 1976). Answer: yes with negligible 

uncertainty. 
TP E-300, Ver. 2.3 February 11, 2019 91

References 

 Richardson, D. M., P. Pyšek, M. Rejmánek, M. G. Barbour, F. D. Panetta, and C. J. West. 

2000. Naturalization and invasion of alien plants: Concepts and definitions. Diversity and 

Distributions 6:93-107. 

 Gordon, D. R. 1998. Effects of invasive, non-indigenous plant species on ecosystem 

processes: Lessons from Florida. Ecological Applications 8(4):975-989. 

Imp-N2 Does the taxon change habitat structure? (y = 0.2, n = 0, or ? = 0)

Definitions 

 Habitat is the “place where an animal or plant normally lives, often characterized by a 

dominant plant form or physical characteristic (that is, the stream habitat, the forest 

habitat)” (Ricklefs and Miller, 2000). 

 Vegetation layer refers to the physical layers of plant communities, such as the 

herbaceous (understory), shrub, and canopy layers of forests and other habitats. 

Rationale. This question strictly addresses changes to the overall structure of habitats, including 

the number and density of vegetation layers. The most obvious changes to habitat structure occur 

when an invading taxon converts a habitat from one type into another (e.g., a prairie into a forest, 

a shrubland into a grassland, or a mudflat into a grassy sward). Less obvious changes include 

effects on the number or density of layers of a given habitat. “Community structure” is 

commonly used synonymously with “habitat structure.” 

Criteria. If a taxon modifies the overall structure of the habitat, creates a new layer, eliminates or 

covers a layer, or significantly modifies the density of a layer, then answer yes. Aquatic taxa that 

form mats on the surface of otherwise open water bodies may also receive a yes for this question. 

Taxa that form dense monotypic stands may also receive a yes if they reduce the structural 

diversity of the habitat. To answer yes for taxa reported to form monotypic stands, we require 

evidence of what the previous habitat structure was like to conclusively determine if it changed. 

If not, evidence of monotypic stands would support an answer of unknown. If no evidence is 

available and the taxon is relatively well known or studied, then answer no. If the taxon is poorly 

known or you suspect it may be having this impact, then answer unknown and provide your 

rationale. 

Full examples 

 Ligustrum sinense alters habitat structure (Smith, 2008); it displaces the native shrub 

layer (Weber, 2003) and often forms monotypic stands (Hanula et al., 2009). Answer: yes 

with negligible uncertainty. 

 Cestrum laevigatum forms dense stands that prevent the regeneration of native shrubs and 

trees (Fourie, 2011; Weber, 2003), but we found no information about this plant changing 

forest structure. Thus, we answered unknown with maximum uncertainty. 

References 

 Ricklefs, R. E., and G. L. Miller. 2000. Ecology (4th ed.). W. H. Freeman and Company, 

New York. 822 pp. 
TP E-300, Ver. 2.3 February 11, 2019 92

Imp-N3 Does the taxon change species diversity? (y = 0.2, n = 0, or ? = 0)

Rationale. Species diversity refers to the numbers of species in a community (species richness) 

and the relative abundances of those species (Ricklefs and Miller, 2000). Thus this question 

addresses changes to the composition of species in natural communities. Invasive plant species 

can dominate communities and alter species composition through any number of mechanisms, 

including direct competition, propagule pressure, and responsiveness to disturbance events. 

Some invasive plant taxa may slowly change the environment to the detriment of natives. 

Although the majority of the available evidence on the impacts of invasive plant species relates 

to impacts on native taxa, this question also considers impacts to animal communities as well. 

Criteria. Answer yes if the taxon changes species diversity or suppresses seedlings of recruiting 

native species, leading to altered species diversity over time. Although it would be preferable to 

understand the exact mechanism(s) by which species composition is altered, that evidence is not 

necessary for a positive answer to this question. If no evidence is available and the taxon is 

relatively well known or studied, then answer no; if the taxon is poorly known or you suspect it 

may be having this impact, then answer unknown and provide your rationale. 

Full examples 

 Delairea odorata reduces biodiversity in state parks in California (Elliott, 1994). In areas 

that have not become monospecific, D. odorata impacts non-native species, and reduces 

native species richness by 50 percent, seedling diversity by about 90 percent, with greater 

impacts on annual than woody species (Alvarez and Cushman, 2002). This taxon 

smothers vegetation in the low scrub and grass/forb level (Csurhes and Edwards, 1998). 

In a study ranking species for their potential to impact biodiversity in New South Wales, 

D. odorata ranked 16th out of 340 species (Downey et al., 2010). Answer: yes with 

negligible uncertainty. 

Imp-N4 Is the taxon likely to affect Federal Threatened & Endangered species? (y = 0.1, n = 

0, or ? = 0)

Rationale. In this question, we are evaluating the likelihood a weed or invasive species may

impact Federal Threatened and Endangered (T&E) species. Often we will not find direct 

evidence for this impact, particularly if the taxon has not been introduced into or is not widely 

distributed in the United States. Thus, this answer relies on expert judgment. It is beyond the 

scope of a routine WRA to do a detailed and specific evaluation for the taxon being evaluated to 

determine which Federal T&E species it may affect. We approach this question at a very broad 

and general level, and assume that most U.S. natural systems contain one or more T&E species 

that may be impacted by an invading taxon. 

Criteria. To answer the question, consider the types of habitats and systems invaded by the taxon, 

and whether it causes any of the impacts described under questions Imp-N1 through Imp-N3. 

Taxa that invade natural systems and cause any of the impacts described in those three questions 

are likely to affect T&E species; in these cases, answer yes, unless specific evidence suggests 

otherwise. Because T&E species are likely to be restricted to undisturbed systems, weeds that 
TP E-300, Ver. 2.3 February 11, 2019 93

only grow in highly disturbed systems (e.g., croplands, cities) are unlikely to affect them, but 

exceptions may exist. This question, unlike most others in this risk element, allows for 

reasonable and defensible speculation on the part of the analyst (hence the term “likely”). If a 

particular species is unlikely to impact T&E species, then answer no or unknown as appropriate. 

Notes 

 In the PLANTS database advanced search engine you can search for T&E species in 

particular states or counties: http://plants.usda.gov/adv_search.html

 The U.S. Fish and Wildlife Service’s Threatened and Endangered Species System 

(TESS) database lists current information on T&E species: 

http://ecos.fws.gov/tess_public/

 Direct evidence for impacts on T&E species in the United States or elsewhere in the 

world reduces uncertainty to negligible. 

Full examples 

 In a model prioritizing 340 invasive weeds, Rumex sagittatus was identified as posing a 

very high threat to biodiversity, ranking 22nd in the list (Downey et al., 2010). It impacts 

the New South Wales Threatened taxon Allocasuarine portuensis (Coutts-Smith and 

Downey, 2006; The University of Queensland, 2013). Answer: yes with negligible 

uncertainty. 

 At least one Federally listed Threatened plant, sea-beach amaranth (Amaranthus pumilus) 

(USFWS, 2005) would be in greater peril if beach vitex (Vitex rotundifolia) invades its 

habitat. The fibrous root system of beach vitex traps endangered sea turtles and impedes 

their nesting (SCBVTF, 2005). Invasion by beach vitex would also disrupt the habitat of 

several Federal Threatened and Endangered birds (Suiter, 2005). Finally, this species 

outcompetes endangered species in the United States (Kaufman and Kaufman, 2007). 

Answer: yes with negligible uncertainty. 

 Solanum sisymbriifolium mainly grows in disturbed areas such as “waste areas, roadsides, 

fence rows, and dykes” (Karaer and Kutbay, 2007), so it seems unlikely that S. 

sisymbriifolium would affect Threatened and Endangered species in natural areas. 

Answer: no with moderate uncertainty. 

Imp-N5 Is the taxon likely to affect any U.S. globally outstanding ecoregion? (y = 0.1, n = 0, 

or ? = 0)

Rationale. Globally outstanding ecoregions are ecologically unique because of higher levels of 

biodiversity, endemism, unique evolutionary phenomena, etc. Ricketts et al. (1999) identified 

these regions in the United States and Canada. 

Criteria. To answer this question, consider the potential distribution of the taxon in the United 

States and the potential habitats it may invade. Determine if the taxon is likely to impact any of 

the globally outstanding ecoregions as defined by Ricketts et al. (1999, p. 34, Fig. 3.1). Taxa that 

are likely to have significant and widespread impacts in these regions receive a yes. Then 

consider the types of impacts the taxon is causing as categorized by Imp-N1 and Imp-N2. If the 

species invades natural systems, can establish in globally outstanding ecoregions, and has 

demonstrated an ability to change habitat structure or ecosystem processes, then answer yes. 
TP E-300, Ver. 2.3 February 11, 2019 94

Although invasive plant species can have significant impacts on species diversity at a local level, 

that type of impact (Imp-N3) is not sufficient to merit a yes because this question’s scope is 

much broader. This question, unlike most others in this risk element, allows for reasonable and 

defensible speculation on the part of the analyst (hence the term “likely”). If a taxon is unlikely 

to impact globally outstanding ecoregions in the United States, then answer no or unknown as 

appropriate. For example, answer no for species that are clearly associated only with disturbed 

environments. 

Full examples 

 Iris pseudacorus is a “potential threat to Louisiana wetlands” (Pathikonda et al., 2009). It 

grows in freshwater wetlands, salt marshes, and riparian habitats (Coops and Van Der 

Velde, 1995; Ramey and Peichel, 2001; Weber, 2003). Based on the impacts listed in 

Imp-N1 through Imp-N3, this plant could alter globally outstanding wetland and riparian 

habitats in the United States where it does not yet occur, such as the Florida Everglades 

(Ricketts et al., 1999). Answer: yes with low uncertainty. 

References 

 Olson, D. M., and E. Dinerstein. 2002. The global 200: Priority ecoregions for global 

conservation. Annals of the Missouri Botanical Garden 89:199-224. 

 Olson, D. M., E. Dinerstein, E. D. Wikramanayake, N. D. Burgess, G. V. N. Powell, E. 

C. Underwood, J. A. D'Amico, I. Itoua, H. E. Strand, J. C. Morrison, C. J. Loucks, T. F. 

Allnutt, T. H. Ricketts, Y. Kura, J. F. Lamoreux, W. W. Wettengel, P. Hedao, and K. R. 

Kassem. 2001. Terrestrial ecoregions of the world: A new map of life on earth. 

BioScience 51(11):933-938. 

 Ricketts, T. H., E. Dinerstein, D. M. Olson, C. J. Loucks, W. Elchbaum, D. DellaSala, K. 

Kavanagh, P. Hedao, P. T. Hurley, K. M. Carney, R. Abell, and S. Walters. 1999. 

Terrestrial Ecoregions of North America: A Conservation Assessment. Island Press, 

Washington D.C. 485 pp. 

Imp-N6 What is the taxon’s weed status in natural systems? Choose one: (a) Taxon not a 

weed; (b) taxon a weed but no evidence of control; (c) taxon a weed and evidence of control 

efforts. (a = 0, b = 0.2, c = 0.6, ? = 0). 

Rationale. Loosely defined, a weed is simply a plant viewed to be undesirable due to some type 

of negative impact. As such, the term weed relates to how people perceive a plant. In this 

question, we are using perception as an index of impact. 

Criteria. Choose the best answer for the taxon being assessed and describe your reasoning or 

your uncertainty as appropriate. Choose your alternate answers for the uncertainty analysis and 

report them as either “The alternate answers for the uncertainty analysis were X and Z” or “The 

alternate answers for the uncertainty analysis were both X.” Answering “unknown” is 

permissible for this question, but that implies that you believe that the three choices, a, b, and c 

are equally likely. That scenario is unlikely because one answer can almost always be ruled out 

or considered to be much less likely than the others. For example, taxon is clearly described as a 

weed, which precludes “a,” or you find no evidence of control, which precludes “c.” One 
TP E-300, Ver. 2.3 February 11, 2019 95

reasonable scenario for an answer of “unknown” is if the taxon being evaluated is new (i.e., 

recently developed cultivar or hybrid), with no prior history of behavior. 

 If a taxon is not considered a weed, then it may not have any specific or 

appreciable impacts in natural areas. Answer “a” if you find no evidence the taxon 

is considered a weed. 

 Answer “b” if the species is listed or considered a weed of natural systems in any 

relevant reference. Our most sensitive measure for impact is how people perceive 

a plant. If a plant is regarded as a weed, then it likely has some type of impact. 

We consider this a sensitive measure of impact because perceptions will usually 

change before published studies and factsheets become available. This measure of 

impact is particularly useful when relatively little published information is found. 

Species that have naturalized in areas set aside for conservation may also be 

considered weeds, because a general goal of conservation efforts is to maintain 

native species diversity. In those cases, use higher uncertainty. 

 We consider any species that is being managed, controlled, or regulated to be 

causing significant harm. Evidence of control efforts is not only an indirect 

measure of impact, but also captures the indirect economic costs associated with 

weed management. A variety of evidence may be used to support an answer of 

“c,” including: 1) direct evidence of management, 2) information linking 

management strategies to the taxon, 3) studies evaluating efficacy of different 

management strategies or herbicide formulations on the taxon, and 4) regulatory 

lists from foreign governments/entities. If a plant is regulated by a foreign entity, 

the evidence must be clear that the taxon is being regulated because of impacts in 

natural systems. Otherwise the same evidence could be used to answer questions 

Imp-N6, Imp-A4, and Imp-P6, which would overinflate the risk score for Impact 

Potential. If it is not clear why the taxon is regulated, then state this in the notes 

field for all three questions without using this to support an answer of “c.” Also, 

weeds are often managed collectively in natural, anthropogenic, or production 

systems. Therefore, answering “c” because a weed is present in an area with 

ongoing weed management for multiple weeds is not sufficient. To answer “c,” 

you must provide evidence that the taxon is specifically targeted or prioritized 

beyond a general weed management program. 

Notes 

 Search taxon name + “control,” “manage,” or “herbicide.” Search regulatory lists. In both 

cases verify that the plant is being controlled in natural systems. 

 A plant in its native range may be considered a weed. This type of information is 

valuable, particularly for plants that have not been taken out of their native range. 

Full example 

 Pistacia chinensis is regarded as an environmental weed in New South Wales and the 

Australian Capital Territory and as a potential environmental weed or 'sleeper weed' in 

other parts of southern Australia (University of Queensland, 2011). It is considered a 

weed of the natural environment in Australia by others as well (Randall, 2011; Csurhes 

and Edwards, 1998) but we found no specific evidence of management. Alternate 

answers for the uncertainty analysis are both “c.” Answer: “b” with moderate uncertainty. 
TP E-300, Ver. 2.3 February 11, 2019 96

 Geranium lucidum is a major weed of natural systems (Dennehy et al., 2011). It can 

invade and overwhelm high quality native habitat, including woodlands and prairies 

(Anonymous, 2013). The Nature Conservancy in Oregon has been trying to eradicate it 

from some of their preserves (Alverson, 2007). It is a specific management target in 

Washington and Oregon in oak woodland, prairie, and savanna habitats within the 

Willamette Valley, Puget Trough, and Georgia Basin ecoregions (Dennehy et al., 2011).

Hand pulling is effective for small populations, but for larger infestations, herbicide 

application at the seedling stage is best (Dennehy et al., 2011). Similar tips for 

management can be found on the King County government website (Anonymous, 2013). 

Alternate answers for the uncertainty analysis are both “b.” Answer: “c” with negligible 

uncertainty. 

Sub-element: Anthropogenic Systems 

This sub-element evaluates impacts the taxon has in anthropogenic systems, which are areas that 

are highly disturbed and populated by people, including cities, suburbs, and transportation and 

utility corridors. Anthropogenic systems categorically exclude all natural and agricultural 

production systems. 

Imp-A1 Does the taxon negatively impact personal property, human safety, or public 

infrastructure? (y = 0.1, n = 0, or ? = 0) 

Rationale. Beyond direct impacts to human health and cultivated plants, some weeds and 

invasive plant species affect human property, processes, civilization, and safety. For example, 

weeds and invasive plant species may affect personal property by staining cars and uplifting 

sidewalks (e.g., Bucida burcera, Morus rubra). They may endanger human safety by 

destabilizing coastal dunes or reducing visibility along roadways (e.g., Vitex rotundifolia, 

Phragmites australis). They may also damage public infrastructure such as canals, sewers, 

bridges, roads, and electrical grids (e.g., Arundo donax, Pueraria lobata). 

Criteria. To answer “yes,” direct evidence is necessary. If the taxon is well studied and you find 

no evidence of these types of impacts, then answer “no.” If the taxon is not well known and you 

suspect it may have these kinds of impacts based on its biology or similarity to other taxa that 

have these impacts, then answer “unknown” and provide your reasoning. 

Notes 

 This question does not apply to plants that may be toxic or allergenic to people. During 

the development and validation of the PPQ WRA, that question was not predictively 

useful in discriminating between invaders and non-invaders in the broad sense). 

However, because impacts to human health are still important in the definition of a 

Federal Noxious Weed, record and convey such impacts elsewhere in the final WRA 

document, either under the summary for the impact risk element, or in the discussion 

section. 

Full examples 

 Coastal ecologists and beach volunteers have noted that Vitex rotundifolia (beach vitex) 

does not retain the sand as well as native vegetation; specifically, they noticed that dune 
TP E-300, Ver. 2.3 February 11, 2019 97

profiles were lower where beach vitex was established (BVTF, 2010). The U.S. Army 

Corp of Engineers, which recently completed a $2 million beach restoration project in 

South Carolina, is concerned about the potential impact of this plant on the stability of 

coastal dunes (Socha and Roecher, 2004). We used moderate uncertainty as this appears 

to be based on casual observation. Answer: yes, with moderate uncertainty. 

 “Planted but soon regretted. Wide-creeping rootstocks (of the timber bamboo 

Phyllostachys aurea) form impenetrable solid stands. 'Roots of steel' can buckle 

sidewalks and driveways” (J.R. Allison, Invasive.org 2012). Answer: yes with low 

uncertainty. 

References 

 Invasive.org. 2012. Invasive.org: Center for Invasive Species and Ecosystem Health, The 

University of Georgia - Warnell School of Forestry and Natural Resources and College of 

Agricultural and Environmental Sciences - Department of Entomology. Available at: 

http://www.invasive.org. 

Imp-A2 Does the taxon change or limit recreational use of an area? (y = 0.1, n = 0, or ? = 0)

Rationale. In addition to the impacts described above under natural systems, plant taxa may 

change or limit the way people use and/or interact with the environment, and usually in a 

negative way. This includes hunting, fishing, sightseeing, navigation, hiking, skiing, and other 

ecotourism-related activities (USFWS, 2006). Invasive plants may change vistas [e.g., Melaleuca 

quinquenervia (Serbesoff-King, 2003)], limit access [e.g., Ligustrum sinense (Hanula et al. 

2009)], or otherwise make an area undesirable. With respect to sightseeing, remember that 

millions of domestic and international visitors travel to national parks to experience natural 

heritage areas and observe unique vistas (USFWS, 2006). Unfortunately, the types of impacts 

captured by this question are often not well studied or reported. 

Criteria. Only direct evidence from the published literature or documented public concerns about 

these types of impacts support yes answers. Personal communications with field experts familiar 

with the taxon could be an important source of information. If the taxon is well studied and you 

find no evidence of these types of impacts, then answer no, otherwise answer unknown as 

appropriate. 

Notes 

 Consider contacting park and recreational area managers. 

 An area may become undesirable if the invading taxon poses a health risk to people or 

pets. 

 Uncertainty is likely to be greater because these types of impacts are not well reported or 

even studied. 

Full examples 

 Arundo donax hampers navigability: “...quite troublesome in many river systems, 

preventing the usual recreation and angling activities” (Moreira et al., 1998). Answer: yes 

with low uncertainty. 
TP E-300, Ver. 2.3 February 11, 2019 98

 Hakea sericea obstructs access and vision (van Wilgen and Richardson, 1985; Wells et 

al., 1986). Has prickly, needle-like, 1 mm-thick leaves that restrict access to mountains 

(CABI, 2012). “Reduces the aesthetic, recreational and scientific value of the indigenous 

plant communities” (cited in Kluge and Neser, 1991). Answer: yes with negligible 

uncertainty. 

References 

 Hanula, J. L., S. Horn, and J. W. Taylor. 2009. Chinese privet (Ligustrum sinense) 

removal and its effect on native plant communities of riparian forests. Invasive Plant 

Science and Management 2(4):292-300. 

 Serbesoff-King, K. 2003. Melaleuca in Florida: A literature review on the taxonomy, 

distribution, biology, ecology, economic importance and control measures. Journal of 

Aquatic Plant Management 41(2):98-112. 

 USFWS. 2006. National survey of fishing, hunting, and wildlife-associated recreation. 

United States Department of the Interior (DOI), U.S. Fish and Wildlife Service 

(USFWS), and U.S. Department of Commerce, U.S. Census Bureau. 168 pp. 

Imp-A3 Does the taxon affect desirable and ornamental plants and vegetation? (y = 0.1, n = 

0, or ? = 0)

Rationale. This question addresses taxa that are in essence nuisance weeds to humans; they may 

not have any impacts in production or natural systems. 

Criteria. Direct evidence that the taxon displaces, outcompetes, or otherwise affects ornamental 

or other desirable plants in gardens, lawns, hedges, cities, aquaria, ponds, etc., is necessary for a 

“yes” response. Usually the best source for information for this question and the other questions 

relating to anthropogenic impacts are gardening forums and blogs. However, because the concept 

of weediness is a perception and often subjective, you should not rely on any single observation 

or comment to support a “yes” response; instead look for multiple independent observations to 

support a “yes” answer. If the taxon is well studied and you find no evidence of these types of 

impacts, then answer “no,” otherwise answer “unknown” as appropriate. 

Notes 

 Impacts to commercially cultivated plants (e.g., farms, nurseries, forests) are considered 

under production system impacts. 

Full examples 

 One gardener on Dave's Garden forum says Delairea odorata is twining into her 

plantings (Dave's Garden, 2013b), but does not explicitly state if it is having any impacts. 

We answered unknown because this was the only evidence we found and because it is not 

clear if there are any actual impacts. Answer: unknown with maximum uncertainty. 

 Of Iris pseudacorus, one gardener wrote, “When it is in its prime habitat of shallow water 

it becomes a thug. It can seed over an immense area crowding out nearly every other 

plant” (Murrain, 2011). Another gardener says, “[this plant] was planted by the previous 

owner around our fish pond....it's dominating and crowding out or hiding other plants I'd 

like to show off” (Dave's Garden, 2013c). Yet another gardener writes, “I, like an idiot, 
TP E-300, Ver. 2.3 February 11, 2019 99

put this in a small pond in our deck without checking it out first. It has a root mass from 

Hell and literally took up every square inch of pond space....I am surprised there are any 

bodies of water in existence with this monster around” (Dave's Garden, 2013c). Answer: 

yes with low uncertainty. 

Imp-A4 What is the taxon’s weed status in anthropogenic systems? Choose one: (a) Taxon 

not a weed; (b) Taxon a weed but no evidence of control; (c) Taxon a weed and evidence of 

control efforts. (a = 0, b = 0.1, c = 0.4, ? = 0) 

Criteria. Follow the guidance provided under Imp-N6, except that the scope here is 

anthropogenic systems. Additional information regarding each answer choice follows. Evidence 

used to support this question is more likely to represent anecdotal comments (e.g., from 

gardeners, homeowners, or city officials). Because those sources of information are not peerreviewed and because anyone can easily claim that a plant is a weed or is being controlled in 

their yard, a greater number of sources or higher quality evidence is required to support “b” or 

“c” responses. 

 For this question, unlike for Imp-N6, simple plant establishment on roadsides or in city 

neighborhoods does not automatically represent weedy behavior. Look for evidence it is 

actually considered a weed in anthropogenic areas to answer “b.” 

 Finding specific evidence for weed control in anthropogenic areas is challenging, because 

such activities are not often well documented. A “c” answer requires direct evidence the 

species is targeted or controlled in anthropogenic areas. Gardening websites are good 

sources to determine if homeowners are taking any actions. Also consider extension 

offices when evaluating lawn weeds, or departments of transportation when evaluating 

roadside weeds. 

Full examples 

 Hakea sericea is a ruderal, and industrial (tourist) weed (Wells et al., 1986). It is one of 

the weeds targeted for removal under the working for water program in South Africa 

(CABI, 2012). While this species is clearly considered an anthropogenic weed and is 

controlled because it affects water availability, the importance of these impacts relative to 

those in natural systems in motivating control is not clear. For this reason we used high 

uncertainty. Both alternate answers for the stochastic simulation were “b.” Answer: “c” 

with high uncertainty. 

 Dittrichia graveolens is present along roadsides, in empty lots, and in disturbed areas in 

California (Univ. of California, 2013), and in similar habitats in Australia (Parsons and 

Cuthbertson, 2001). It is being controlled along roadsides in Australia (Kay, 1981). In 

California, it is primarily a weed of roadsides and highly disturbed areas, and different 

control strategies are being tested (Brownsey et al., 2013). It is treated by the California 

Department of Transportation along roadsides (Ortiz, 2013). Alternate answers for the 

stochastic simulation were both “b.” Answer: “c” with negligible uncertainty. 

Sub-element: Production Systems 

This sub-element evaluates impacts the taxon has in production systems, which are areas of 

agricultural or forestry production including row crops, orchards, plantations, pastures, 

rangelands, aquaculture, nurseries, etc. Some production systems, such as rangelands (but not 
TP E-300, Ver. 2.3 February 11, 2019 100

pastures) and national forests, may also be considered natural systems because they are 

maintained or managed in a near pristine or wild state. In those cases, consider and distribute the 

evidence as appropriate across each system, by type of impact. For example, a taxon that invades 

rangelands may change habitat structure (natural system impact), but may also reduce the 

carrying capacity of the land to support cattle (production system impact). 

Imp-P1 Does the taxon reduce crop or commodity yield? (y = 0.4, n = 0, or ? = 0) 

Rationale. Weeds reduce yield by competing with crops for space, nutrients, light, and water 

resources. Some weeds directly damage crops through parasitism or other mechanisms. They 

may also indirectly increase pest loads on cultivated species if the weed is a host or alternate host 

for a pest. All of these mechanisms may result in reduced crop and commodity yield, particularly 

if the weeds are abundant enough. 

Criteria. Direct evidence of yield reduction (quantitative or qualitative) is necessary for a yes 

response. Evidence for yield reduction in pasture productivity may be more difficult to obtain, 

but direct evidence is nevertheless required. In these cases, evidence that the carrying capacity of 

pastures is reduced, or that animal stock avoid the plants and therefore promote weed abundance 

indirectly is sufficient for a yes response. If the taxon is well studied and you find no evidence of 

these types of impacts, then answer no, otherwise answer unknown as appropriate. If yield loss is 

suspected, but no evidence is available, answer “unknown” and provide your reasoning. 

Quantitative data on yield loss should reduce uncertainty. 

Notes 

 Search in CAB Abstracts, the CABI Crop Protection Compendium, or the CABI Forestry 

Compendium for information on weed impacts in production systems. 

Full examples 

 Nassella neesiana can account for up to 60 percent of canopy cover in infested pastures 

(Gardener et al., 2003). Because flowering stems are not palatable (Bourdôt et al., 2012), 

plant populations greatly reduce stock-carrying capacity during the summer (Gardener et 

al., 2003; Snell and Grech, 2008), but they are a good source of forage during the winter. 

Flowering stalks are actively avoided by stock (Grech et al., 2006). Answer: yes, with 

negligible uncertainty. 

 In cut-over coastal redwood forests in northern California, Cortaderia jubata suppresses 

establishment of seedling conifers (Bossard et al., 2000). In forestry operations, it is very 

competitive and can retard the establishment and growth of seedling trees (ODA, 2013). 

Answer: yes, with moderate uncertainty. 

Imp-P2 Does the taxon lower commodity value? (y = 0.2, n = 0, or ? = 0) 

Rationale. A plant taxon may lower commodity value by deforming/damaging plant or animal 

commodities, contaminating seed lots, or otherwise altering commodity quality. For example, 

some pasture weeds when consumed by cattle produce an off-flavor to the milk. Weed seed 

contaminants of commercial seed usually necessitate cleaning procedures, or reduce sale price 

because of lowered quality. Weed taxa may also lower commodity value by increasing the costs 
TP E-300, Ver. 2.3 February 11, 2019 101

of production due to weed management or other costs to machinery. For example vines that form 

tangles could interfere with harvesting equipment in row crops and orchards, and slow down 

harvest. 

Criteria. Direct evidence is required for a yes response. If the plant is well known and you find 

no direct evidence, then answer no. Otherwise answer unknown, as appropriate. 

Notes 

 Search in CAB Abstracts, the CABI Crop Protection Compendium, or the CABI Forestry 

Compendium. 

Full examples 

 Seeds of Nassella neesiana have a very sharp point (Richardson et al., 2006). The sharp 

seeds bore into animal skins, causing painful wounds (Haywood and Druce, 1919). 

Sometimes seed will pierce the skin of sheep, irritating the animal, damaging the hide, 

and reducing its value (Gardener et al., 2003; Storrie and Lowien, 2003). “Its invasion of 

these grasslands also leads to the downgrading of wool, skins (hides) and carcasses as a 

result of the sharp callus and hygroscopic geniculate awn that together facilitate 

penetration of the mature fruit into the wool, skin and underlying muscle of grazing 

animals” (Bourdôt, 2010; Bourdôt et al., 2012). Answer: yes with negligible uncertainty. 

Imp-P3 Does the taxon impact or is it likely to impact trade? (y = 0.2, n = 0, or ? = 0) 

Definitions (from IPPC 2013): 

 Definition of “quarantine pest”: A pest of potential economic importance to the area 

endangered thereby and not yet present there, or present but not widely distributed and 

being officially controlled (IPPC, 2013). 

 Definition of “regulated, non-quarantine pest”: A non-quarantine pest whose presence in 

plants for planting affects the intended use of those plants with an economically 

unacceptable impact and which is therefore regulated within the territory of the importing 

contracting party (IPPC, 2013). 

Rationale. A weedy taxon is likely to impact trade if it is a quarantine pest or a regulated nonquarantine pest (see definitions below) that can follow the pathway of a traded commodity. In 

these cases importing countries or administrative regions within those countries may destroy or 

reject contaminated shipments, or they may impose phytosanitary treatments to reduce the risk 

associated with the contaminant. 

Criteria. Answer yes if you find direct evidence that a plant is a regulated pest and it can follow a 

pathway of a traded commodity. This question allows for reasonable speculation by the analyst. 

You need not establish both criteria—i.e., that the pest is regulated and a pathway exists—for 

any given country or region (e.g., state or province) because that is usually not available. If the 

taxon is regulated anywhere in the world, and if it can follow a pathway on any traded 

commodity, answer yes. Answer “no” if the taxon is unlikely to follow a trade pathway (e.g., 

coconut seeds). The answer to the pathway component of this question should be consistent with 
TP E-300, Ver. 2.3 February 11, 2019 102

the answer given under ES-16. If the plant is well known, and you find no direct evidence then 

answer no; otherwise answer unknown, as appropriate. 

Notes 

 Search the internet: taxon name + “quarantine” + “regulat*” + contaminant + treatment. 

Some countries refer to regulated weeds as “noxious weeds” (e.g., Australia; Parson and 

Cuthbertson, 2001) or “declared weeds” (South Africa; Henderson, 2001). 

 If accessible, search for governmental lists of regulated pests (e.g., APHIS, 2014.). For 

U.S. regulated weeds, consider not only the Federal Noxious Weed (7 CFR §361) and 

Federal Noxious Seed lists (7 CFR §361), but also State noxious weed and noxious seed 

lists (USDA-AMS, 2013). 

 When the weed taxon is the commodity and is intentionally introduced (e.g., plants for 

planting) answer no, unless you have evidence the taxon is a contaminant in some other 

pathway where it is not being intentionally introduced. 

 If the commodity being introduced contains its own viable propagules (e.g., seeds in 

dried flower arrangements or in pine cone decorations) then answer yes. 

Full examples 

 Praxelis clematidea is a declared noxious weed in Western Australia; consequently, it is 

not permitted entry and must be eradicated when found in the state (DAFWA, 2012). 

Because it is a contaminant of seeds moving in trade (Waterhouse, 2012; Scott, 1998) and 

of other commodities and conveyances (AQAS, 2014), it may impact trade to areas 

restricting its movement. This species spreads in sugar cane mulch (English, 2014), 

which is restricted entry into Western Australia because of weed contaminants (DAFWA, 

2013). Answer: yes with negligible uncertainty. 

References 

 APHIS. 2014. Phytosanitary Certificate Issuance & Tracking System (PCIT). United 

States Department of Agriculture, Animal and Plant Health Inspection Service (APHIS). 

https://pcit.aphis.usda.gov/pcit/faces/index.jsp. 

 Henderson, L. 2001. Alien Weeds and Invasive Plants: A Complete Guide to Declared 

Weeds and Invaders in South Africa. Agricultural Research Council, South Africa. 300 

pp. 

 IPPC. 2013. International Standards for Phytosanitary Measures No. 5: Glossary of 

Phytosanitary Terms. Food and Agriculture Organization of the United Nations, 

Secretariat of the International Plant Protection Convention (IPPC), Rome, Italy. 

 Parsons, W. T., and E. G. Cuthbertson. 2001. Noxious weeds of Australia (2nd edition). 

CSIRO Publishing, Collingwood, Victoria, Australia. 698 pp. 

 USDA-AMS. 2013. State noxious-weed seed requirements recognized in the 

administration of the Federal Seed Act. United States Department of Agriculture 

(USDA), Agricultural Marketing Service (AMS), Washington D.C. 116 pp. 
TP E-300, Ver. 2.3 February 11, 2019 103

Imp-P4 Does the taxon affect the quality or availability of irrigation, or strongly compete 

with plants for water? (y = 0.1, n = 0, or ? = 0)

Rationale. Although all plants compete with each other for water and other resources, the scope 

of this question is limited to taxa that are exceptionally competitive for water or that affect the 

quality or availability of water (e.g., by clogging irrigation canals). Adequate water resources are 

critical in production systems. 

Criteria. Direct evidence is required for a yes, and it must demonstrate why the taxon is 

exceptional in this regard. Also answer yes if the taxon being evaluated contributes to flooding or 

hinders drainage in production systems. If the plant is well known, and you find no direct 

evidence then answer no, otherwise answer unknown, as appropriate. 

Notes 

 This question applies only to production systems. Impacts on water quality or availability 

that affect natural resources or humans are considered above under natural and 

anthropogenic systems. 

 Taxa that affect navigation in anthropogenic systems or water resources in natural 

systems will not necessarily affect water resources in production systems. Specific 

evidence of impact in production systems is required to support a yes. 

Full examples 

 Luziola subintegra impedes the flow of water to irrigated rice fields and increases the 

evapotranspiration of water, which is valuable for rice production (Fischer, 1997). It is 

associated with irrigation canals (and/or rice fields) in Costa Rica (Rojas and Agüero, 

1996), but those authors did not state whether or not it had a significant effect on rice 

irrigation. Answer: yes with low uncertainty. 

 In tea production, Persicaria chinensis is a common weed often found in field drainage 

channels (Barbora, 1972). This is one of a group of weed species described as restricting 

the free flow of surplus water that reduces the efficiency of an otherwise adequate 

drainage system (Barbora, 1972). Because we are uncertain if P. chinensis or the other 

weed taxa affect water in tea production, we used high uncertainty. Answer: yes with 

high uncertainty. 

Imp-P5 Is the taxon toxic to animals? (y = 0.1, n = 0, or ? = 0) 

Rationale. Consumption of toxic plants can sicken rangeland and farm animals, cause or increase 

their susceptibility to disease, and, in some cases, cause mortality. Many toxic plants are not 

normally eaten by animals (e.g., Heracleum mantegazzianum). 

Criteria. To answer yes, the animal must reasonably be likely to be exposed to the toxic agent by 

consumption or other physical contact. Toxic plants may grow naturally in pastures and 

rangelands or be contaminants in hay or other feed. Some mildly toxic but palatable plant taxa 

could cause problems if heavily grazed. Consider grazing by both wild and domestic vertebrates. 

If you find direct evidence that consumption of the taxon affects animal health, answer yes. Also 

answer yes if toxic compounds are uniformly characteristic of the plant genus or family, even if 
TP E-300, Ver. 2.3 February 11, 2019 104

data on the particular taxon are not available (but increase uncertainty as this represents 

congeneric information). A lack of positive evidence for this question results in a no or unknown 

answer depending on the amount of information available on the taxon and how well studied it 

may be. 

Notes 

 Some plant taxa cause physical injury to animals (e.g., lacerations, eye punctures). These 

kinds of impacts are beyond the scope of this question which focuses on toxicity per se. 

Evidence of physical injury should be recorded in either the summary paragraph of the 

impact or discussion sections of the weed risk assessment. 

 Some taxa become toxic when they are associated with micro-organisms (e.g., fungal 

endophytes). Note that in these cases, the taxon itself is not toxic, it or its product 

becomes toxic due to improper cultivation, handling, or processing. Generally, in these 

cases, answer no, but use higher uncertainty if you deem necessary. 

 This question focuses on plant toxicity to animals. Although you might include evidence 

of human toxicity to help support your answer, the answer should not be based solely on 

human toxicity evidence. 

 We identify several useful references and databases below. 

Full examples 

 Rumex sagittatus is a health concern for sheep and goats in South Africa (Wells et al., 

1986). Rumex sagittatus is toxic (Randall, 2012). No known risk of toxicity to goats in 

Australia but highly palatable to them (Simmonds et al., 2000). We did not find any other 

information on R. sagittatus. Some species of Rumex are known to be toxic to animals 

and humans, and yet some are consumed with no adverse effects (Burrows and Tyrl, 

2001). Answer: yes with high uncertainty. 

 We found no evidence that Falcaria vulgaris is toxic. Cattle will consume it and humans 

eat it as a vegetable in the Middle East (Korman, 2012). Answer: no with low 

uncertainty. 

References 

 Brown, D. 2011. Plants Poisonous to Livestock (Online Database). Cornell University, 

Department of Animal Science. http://www.ansci.cornell.edu/plants/index.html. 

(Archived at PERAL). 

 Burrows, G. E., and Tyrl. 2001. Toxic Plants of North America. Iowa State University 

Press, Ames, IA, U.S.A. 1342 pp. 

 Cooper, M. R., and A. W. Johnson. 1984. Poisonous Plants in Britain and Their Effects 

on Animals and Man. Her Majesty's Stationery Office, London. 305 pp. 

 USNLM. 2014. PubMed [Online Database]. United States National Library of Medicine 

(USNLM) and the National Institutes of Health. http://www.ncbi.nlm.nih.gov/pubmed. 

(Archived at PERAL). 

 USNLM. 2014. TOXNET: Toxicology data network [Online Database]. United States 

National Library of Medicine (USNLM). http://toxnet.nlm.nih.gov/. 
TP E-300, Ver. 2.3 February 11, 2019 105

Imp-P6 What is the taxon’s weed status in production systems? Choose one: (a) Taxon not 

a weed; (b) Taxon a weed but no evidence of control; (c) Taxon a weed and evidence of 

control efforts. (a = 0, b = 0.2, c = 0.6, ? = 0)

Criteria. Follow the same guidance provided under Imp-N6, except the scope here is production 

systems. For an answer of “b,” the taxon must be recognized as a weed and not merely be present 

in production systems. Studies that evaluate the efficacy of particular herbicide formulations 

against the plant taxon can be used to support an answer of “c.” 

Notes 

 Agricultural databases such as CAB Abstracts and Agricola may be useful in finding 

specific evidence. Likewise, searching Google Scholar for the taxon and “competition,” 

“yield loss” or “control” may provide useful information. 

Full examples 

 Echinochloa pyramidalis is a weed in rice fields in Australia, India, the Philippines, and 

Tropical America (López Rosas et al., 2006). It has been introduced to India and tropical 

America, but apparently is not a significant weed outside Africa (Michael, 1983). It is 

controlled in rice in Madagascar (Bruyere and Rakotomanana, 1964). This species is 

considered a troublesome weed in irrigation canals of the Guyana Sugar Corporation and 

is controlled with herbicides (Bishundial et al., 1997). Along with other weeds, herbicides 

are used to control it in rice (Jauffret, 1954). It is recommended that E. pyramidalis be 

replaced with another grass species along sugarcane irrigation canals in Madagascar 

(Rochecouste, 1965). Alternate answers for the stochastic simulation were both “b.” 

Answer: “c” with low uncertainty. 
TP E-300, Ver. 2.3 February 11, 2019 106

Appendix G. Question-specific guidance and examples for the risk element 

Geographic Potential 

This risk element evaluates the ability of the taxon to survive at different levels of three climatic 

variables: 1) plant hardiness, 2) climate class, and 3) mean annual precipitation. The PPQ WRA 

uses this information two different ways. First, we use it to evaluate the regions of the United 

States where the taxon is likely to survive without human intervention using a simple climate 

matching algorithm developed specifically for the PPQ WRA (see Process 4). Although the 

information is used to predict suitable regions in the United States, the same algorithm can be 

used for other regions of the world. Second, we also use the information to estimate the adaptive 

potential of the taxon as the number of hardiness zones, climate types, or mean annual 

precipitation bands suitable for survival. It has been shown that species with greater climatic 

tolerances are more likely to become invasive than those with narrower tolerances (Sexon et al., 

2002). During the development and validation of the PPQ WRA model, we determined that our 

estimates of the adaptive potential of a species are predictive of weed risk (Koop et al., 2012). 

These three estimates are incorporated into the predictive WRA model in questions ES- 21 

through ES-23. 

This risk element contains 36 questions about the different levels (i.e., sub-classes) of the 

following three variables: Plant Hardiness Zones (13 questions), Köppen-Geiger climate classes 

(12 questions), and mean annual precipitation bands (11 questions). Each question asks if the 

given climatic level is suitable for species survival. All questions require either a yes or no 

answer. Answers of unknown are not acceptable because the process requires each climate level 

to be classified as either suitable or not. 

Unlike in the previous risk elements, where the answers generated scores, we use the geographic 

potential evaluation to produce a map showing the predicted areas for a taxon. 

General guidance for recording evidence 

Most of the evidence we consider for this risk element consists of georeferenced data and reports 

about establishment of a taxon in a particular area (without specific geographic coordinates). As 

described in Process 4, we download these data from the GBIF database and create datasets or 

layers representing global distribution in ArcGIS. Other types of evidence come from literature 

reviews and may include descriptions of climatic tolerances (e.g., evidence that a taxon cannot 

survive temperatures below freezing). Any data relating to the location of established 

populations or climatic tolerance of the taxon is important to record, and may help you answer 

the questions with lower uncertainty. 

As you answer each question, provide your evidence and cite your sources in the Evidence 

column of the “Screening Tool” worksheet. List the countries (states, provinces, etc.) where the 

taxon occurs that support a yes answer. Because most data will be based on GBIF entries, you do 

not need to cite GBIF each time; there is a general comment at the beginning of this section 

stating that uncited information was derived from GBIF6

. All other data sources should be 

 

6

 “Unless otherwise indicated, the following evidence represents geographically referenced points obtained from the 

Global Biodiversity Information Facility (GBIF, 2014).” 
TP E-300, Ver. 2.3 February 11, 2019 107

referenced, along with the location of the taxon in the Evidence column. When you use nonGBIF evidence, note whether it is georeferenced or not. 

General guidance for answering the questions 

When evaluating the evidence to answer each question, consider the total number of 

georeferenced points and reports of occurrence, as well as the overall distribution of the taxon 

and in which particular climates/habitats it can survive. If a taxon has many georeferenced points 

for a given climatic level, answer yes with negligible uncertainty. If a taxon has very few points 

in a level, and you found no other supporting information, then a yes answer is reasonable if it is 

consistent with the biology and native range of the taxon (but raise the uncertainty). Otherwise, a 

yes answer is unlikely. If the taxon has one point in a distinct climate type (e.g., desert), with no 

other evidence to support its establishment there, answer no and raise the uncertainty. For 

example, most broadleaved species are unlikely to survive in very dry habitats. In difficult cases, 

provide an explanation for your answer choice. 

Occurrences in a given climate level do not always mean a taxon can survive in that climate or 

even occur there. For example, although infrequent, some data may represent misidentifications. 

Also, georeferenced data may have been entered incorrectly or points may represent herbaria or 

living collections. Some data might be real occurrences, but in highly protected, modified or rare 

microhabitats. For suspect points, the locality column of the original GBIF data may describe the 

area from which the specimen was taken. Ignore any points that cannot be reconciled with what 

you know about your taxon. We can often remove erroneous georeferenced points (e.g., reversed 

latitudes and longitudes, mismatches between reported country and latitude/longitude) through 

our data cleaning process, described in the Geographic Potential Instruction Manual. 

Consider other factors that may introduce error into your analyses and maps. For example, some 

areas reflect highly variable climates, such as mountainous regions where elevation changes over 

short spatial scales cause significant climatic differences. Georeferenced points in such regions 

may be difficult to interpret. Also the broad scale of our datasets (grid cell size = 10 square km) 

limits our precision. If a point of occurrence falls on a line between two levels (e.g., hardiness 

zones), consider the other points representing your taxon. If six points occur in one level and 

only one (or half of a point) occurs in a new level, include the new level only if other points 

occur in that level elsewhere in the world. 

General guidance for addressing uncertainty 

Use the standard ratings for uncertainty for each yes or no answer. In general, if you found 

sufficient georeferenced data points for a given level and the biology of your taxon matches the 

climate of that area, use negligible uncertainty. If your answer was based only on occurrence 

information in a country, state or province, and multiple climate levels occur in that area, raise 

uncertainty to moderate or high. As mentioned above, in the evidence column of the spreadsheet, 

distinguish between evidence based on georeferenced points and reported occurrences. 

Specific guidance for each climatic variable 

The following is general and specific guidance for answering the geographic questions and 

addressing specific issues that may arise. For each of these three climatic variables, our datasets 

were created on a 10-km scale (each pixel represents 10 square kilometers). 
TP E-300, Ver. 2.3 February 11, 2019 108

Plant Hardiness Zones 

Rationale. Plant Hardiness Zones are based on average yearly minimum temperatures (USDAARS, 2012) and are widely used by horticulturalists to broadly identify regions of the United 

States suitable for plant establishment. The global Plant Hardiness Zone maps we use here are 

based on extreme minimum temperatures in each pixel of our datasets with the values converted 

to plant hardiness zones using USDA definitions (USDA-ARS, 2012). There are 13 global Plant 

Hardiness Zones; the United States and its territories include all 13 (Saha et al. 2010; Magarey et 

al. 2008). The Zones are defined as follows: 

Zone 1 (below -50 °F or below -46.6 °C) 

Zone 2 (-50 to -40 °F or -45.6 to -40.0 °C) 

Zone 3 (-40 to -30 °F, or -40.0 to -34.4 °C) 

Zone 4 (-30 to -20 °F, or -34.4 to -28.9 °C) 

Zone 5 (-20 to -10 °F, or -28.9 to -23.3 °C) 

Zone 6 (-10 to 0 °F, or -23.3 to -17.8 °C) 

Zone 7 (0 to 10 °F, or -17.8 to -12.2 °C) 

Zone 8 (10 to 20 °F, or -12.2 to -6.7 °C) 

Zone 9 (20 to 30 °F, or -6.7 to -1.1°C) 

Zone 10 (30 to 40 °F, or -1.1 to 4.4 °C) 

Zone 11 (40 to 50 °F, or 4.4 to 10 °C) 

Zone 12 (50 to 60 °F, or 10 to 15.6 °C) 

Zone 13 (above 60 °F, or above 15.6 °C) 

Criteria. Answer yes if you find good evidence for the presence of the taxon in that zone. 

Otherwise answer no. If you have evidence for a series of zones, e.g., 5, 6 and 7, and evidence 

for Zones 9 and 10, but not for Zone 8, answer “yes” for Zone 8, based on the evidence above 

and below it. 

Full examples 

 For Zone 6, numerous georeferenced data points in the United States (Kansas, Missouri, 

and Ohio). Answer: yes with negligible uncertainty. 

 A few dozen georeferenced data points exist for Ardisia crenata in Zone 7 clustered in or 

southwest of Tokyo Japan; but given this plant’s cold sensitivity and the heat island effect 

of major cities, we are assuming that it cannot generally live in this hardiness zone. The 

coldest zone this species is reported suitable for is Zone 8 (DavesGarden, 2012; Page and 

Olds, 2001). Answer: no with high uncertainty. 

Köppen-Geiger climate classes 

Rationale. The Köppen-Geiger climate classification scheme has been in use for over 100 years. 

A new global map was produced recently using the Köppen-Geiger system based on a large 

global data set of long-term monthly precipitation and temperature station time series (Peel et al., 

2007), with the global map made available online. We aggregated the third-tier classifications in 

the Peel et al. (2007) dataset (shown in parentheses), to produce the following 12 climate classes: 

Tropical rainforest (Af, Am) 
TP E-300, Ver. 2.3 February 11, 2019 109

Tropical savanna (Aw) 

Steppe (Bsh, Bsk) 

Desert (Bwh, Bwk) 

Mediterranean (Csa, Csb) 

Humid subtropical (Cwa, Cfa) 

Marine west coast (Cwb, Cwc, Cfb, Cfc) 

Humid continental warm summers (Dsa, Dwa, Dfa) 

Humid continental cool summers (Dsb, Dwb, Dfb) 

Subarctic (Dsc, Dsd, Dwc, Dwd, Dfc, Dfd) 

Tundra (ET) 

Icecap (EF) 

Criteria. For each particular climate class, answer yes if you find good evidence for the presence 

of the taxon in that class. Unlike for the plant hardiness and precipitation variables, do not 

automatically answer yes for a climate class that falls between two other climate classes for 

which you have evidence. Answer no if you find no evidence for occurrence within a particular 

class. 

Typically, the most problematic climate class to answer is “Humid continental warm summers” 

(HCWS). Globally, this climate class is present in patchy areas in Europe and Asia, but in the 

United States, it comprises a large portion of many central states (Kansas, Illinois, Indiana, Iowa, 

Missouri, Nebraska, New Jersey, Ohio, Pennsylvania, and South Dakota). Answering yes for 

Humid subtropical (HS) and Humid continental cool summers (HCCS), but no for HCWS, 

results in a predictive map without the above-listed states but with states both north and south. If 

you answer yes for the HS and HCCS climates, we recommend carefully examining the literature 

to find evidence for HCWS in Europe and Asia. Often, searching for the taxon and a particular 

European or Asian country (Slovakia or Japan) will provide the evidence needed. 

Full examples 

 For Mediterranean habitats, numerous georeferenced points exist for Rumex sagittatus in 

Australia and South Africa. Answer: yes with negligible uncertainty. 

 Leptochilus pteropus is broadly distributed in southern India (Singh et al., 2012), which 

includes steppe climates. However, it is unclear if it can occur in steppe climates, as it is 

well adapted to wet habitats. Answer: no with high uncertainty.

Mean annual precipitation bands 

Rationale. The mean annual precipitation maps are based on mean annual precipitation values in 

each pixel across 10 or 30 years (see Process 4). Mean values are expressed as 10-inch (or 25-

cm) precipitation bands, and there are 11 bands as follows: 

 0-10 inches (0-25 cm) 

 10-20 inches (21-51 cm) 

 20-30 inches (51-76 cm) 

 30-40 inches (76-102 cm) 

 40-50 inches (102-127 cm) 

 50-60 inches (127-178 cm) 
TP E-300, Ver. 2.3 February 11, 2019 110

 60-70 inches (152-178 cm) 

 70-80 inches (178-203 cm) 

 80-90 inches (203-229 cm) 

 90-100 inches (229-254 cm) 

 100+ inches (254+ cm) 

Criteria. As before, answer yes if you find good evidence for the presence of the taxon in that 

precipitation band, and otherwise answer no. As with plant hardiness zones, if you find strong 

evidence that a species occurs across a range of bands, except for one band in the middle of the 

range (e.g., 40-50, 50-60 and 70-80, but not 60-70), answer yes for that middle band with the 

same uncertainty level used in the adjacent bands. However, in this example, if the evidence for 

band 70-80 were very weak, consider answering no for bands 60-70 and 70-80. In some regions, 

many bands fall within a given area. In those cases, before answering yes for all of the bands, 

consider the overall distribution of the taxon including other regions of the world. For example, 

if most points are located in areas with 40 to 70 inches of precipitation, but one point occurs 

between 10 and 20 inches, answer “no” for the 10-20 inch band. 

If your taxon is a submerged aquatic species, consider answering “yes” to all or most 

precipitation questions as the distribution for that taxon will not be determined by precipitation 

per se, but rather the availability of aquatic bodies such as ponds, lakes, streams, etc. 

Full examples 

 For the 30-40 inch band, Xanthoceras sorbifolium is reported to occur in areas of China 

(e.g., Gansu, Shanxi, Shandong; NGRP, 2013; Zhengyi et al., 2013) that include this 

precipitation band. Answer: yes with low uncertainty. 

 Multiple points for Falcaria vulgaris occur in Armenia, Azerbaijan, Turkey, Israel, and 

the United States (South Dakota) for the 10-20 inch band. It also occurs in a steppe 

region of Russia receiving an average of 334 mm per year (Yunusbaev et al., 2003). 

Answer: yes with negligible uncertainty. 

References for Geographic Potential 

Koop, A., L. Fowler, L. Newton, and B. Caton. 2012. Development and validation of a weed 

screening tool for the United States. Biological Invasions 14(2):273-294. 

Peel, M. C., B. L. Finlayson, and T. A. McMahon. 2007. Updated world map of the KöppenGeiger climate classification. Hydrology and Earth Systems Sciences 11:1633-1644. 

Sexon, J. P., J. K. McKay, and A. Sala. 2002. Plasticity and genetic diversity may allow saltcedar 

to invade cold climates in North America. Ecological Applications 12(6):1652-1660. 

USDA-ARS. 2012. Plant Hardiness Zone Map, 2012. U.S. Department of Agriculture (USDA), 

Agricultural Research Service (ARS). http://planthardiness.ars.usda.gov. 
TP E-300, Ver. 2.3 February 11, 2019 111

Appendix H. Question-specific guidance and examples for the risk element 

Entry Potential 

This risk element evaluates the potential that a plant taxon will be introduced into the United 

States intentionally for cultivation, accidentally as a contaminant, or through natural dispersal. 

The entry potential risk element consists of 14 questions that evaluate the number of pathways 

with which a taxon is or may be associated. Taxa that are cultivated elsewhere or are positively 

valued by society are more likely to enter than other less valued taxa. The risk score for this 

element ranges from zero to one, with higher scores indicating a higher likelihood of entry. If a 

taxon is already present in the United States, or if a permit for its entry has been requested, 

assessment of this risk factor is not necessary. Otherwise, the question scores for the various 

pathways are summed to produce a risk element score that can range from 0 to 1. Because 

intentional introduction automatically results in the entry of an organism, this question is 

weighted much more highly than the other pathways. Potential score values for each question is 

indicated below in parentheses. 

For each question in the guidelines below, we provide the full question, answer choices, and 

scoring in bold type. Following that is the question-specific guidance. When appropriate, 

additional suggestions, search criteria, and references are provided in bullets. For each question, 

one or more examples for weed risk assessment are provided. 

Ent-1 Is the plant taxon already in the United States? (y=1, n=0)

Criteria. If the plant taxon is already cultivated or naturalized in the United States, answer yes for 

Ent-1, and stop further assessment in this risk element. Many plant collectors specialize in 

growing rare or uncommon plants, so many taxa may be present in the United States but not 

widely cultivated. If you find no indication a species is present in the United States after a 

reasonable internet search, answer no, but use moderate uncertainty. If a plant is present but only 

under quarantine or some other restrictive permit, answer no, because permit restrictions provide 

safeguards that equate to non-presence. 

Notes 

 An answer of yes to either Ent-1 or Ent-2 causes the workbook to automatically set all 

other answers to not applicable, preventing analysts from evaluating the remainder of the 

risk element. If the taxon is present in the United States but evaluating its entry potential 

is still prudent or necessary for some reason, answer no to this question but explain both 

that the taxon is present and why evaluating its entry potential is nevertheless important. 

 Refer to Section 3, Process 2, Section 2-4 of these WRA Guidelines for recommended 

sources for determining whether a taxon is cultivated, naturalized, or under regulations in 

the United States. 

 Check BONAP (http://www.bonap.org/), the USDA Plants database 

(http://plants.usda.gov/java/), EDDMapS (http://www.eddmaps.org/species/), and GBIF 

(http://data.gbif.org/welcome.htm) for U.S. distribution information. 

 Search gardening compendia such as Hortus Third (Bailey and Bailey, 1976). 
TP E-300, Ver. 2.3 February 11, 2019 112

 Search gardening websites such as Dave’s Garden (http://davesgarden.com/), GardenWeb 

(http://forums.gardenweb.com/forums/), Plant Information Online

(http://www.plantinfo.umn.edu/), and the Plant Finder 

(http://www.backyardgardener.com/Plant-Index/). 

 Search online and print nursery catalogues. 

 Try an internet search for “buy + taxon name.” 

Full examples 

 Toona sinensis has been cultivated and traded in the United States for a long time, e.g., 

100 years in California (Grotkopp and Rejmánek, 2007). It is for sale in California 

(smgrowers.com, 2011), Oregon (greergardens.com, 2011), growing in Maryland 

(Tasker, 2011), and escaped in Pennsylvania (Soloman and Dijois, 2011). It is also 

reported to be growing in arboretums in Massachusetts, Pennsylvania, New Jersey, 

Illinois, Washington DC, New York, and Ohio (Koller, 1978), as well as growing (in 

gardens) in Iowa, Kentucky, Mississippi, New York, Pennsylvania, and Texas 

(DavesGarden, 2011). This taxon was acquired by the J. C. Ralston Arboretum in North 

Carolina (JCRA, 1992). Answer: yes with negligible uncertainty.

References 

 Bailey, L. H., and E. Z. Bailey. 1976. Hortus Third: A Concise Dictionary of Plants 

Cultivated in The United States and Canada (revised and expanded by The Staff of the 

Liberty Hyde Bailey Hortorium). Macmillan, New York, U.S.A. 1290 pp. 

Ent-2 Is the plant taxon proposed for import or is its entry imminent? (y=1, n=0)

Criteria. Answer yes for taxa that have been proposed for entry or cultivation in the United States 

and cite relevant supporting documentation, and STOP further analysis in this risk element. If 

you answer no, continue with questions Ent-3 through Ent-5. 

Full example 

 Echinodorus uruguayensis is proposed for import into the United States from Denmark 

(MFAF, 2009). Answer: yes with negligible uncertainty. 

Ent-3 What is the cultivation status of this plant taxon? Choose the best answer: (a) 

Neither cultivated nor positively valued; (b) Not cultivated, but positively valued or 

potentially beneficial; (c) Cultivated, but no evidence of trade or resale; (d) Commercially 

cultivated or other evidence of trade or resale. (a = 0, b = 0.05, c = 0.25, d = 0.5) 

Rationale. Most plants introduced to new areas are intentionally introduced for cultivation. 

Choices “a” through “d” represent increasing levels of entry potential because of potential 

cultivation. You should consider both legal and illegal pathways in this question. 

 Taxa that receive an answer of “a” are the least likely to be intentionally introduced for 

cultivation because they are not used by humans or even considered to be beneficial. 

 Choice “b” represents taxa that are not cultivated, but may be harvested in the wild by 

some people. They may have useful traits or beneficial biochemical properties, and may 

be imported for research purposes. 
TP E-300, Ver. 2.3 February 11, 2019 113

 Choice “c” represents taxa that are cultivated either locally or by a small group of 

specialists, and demand is not large enough for these plants to be traded at any 

appreciable level. 

 Choice “d” represents plants that are easily available for order over the internet or 

through seed or other nursery catalogues. 

Criteria. Answer “a” if you find no evidence of cultivation or potential benefit. Answer “b” if 

you find good evidence for potential benefit, but no evidence of cultivation. Answer “c” if you 

find good evidence for limited cultivation (e.g., research, early breeding trials). Answer “d” if 

you find good evidence for wide cultivation by many different people, including being for sale. 

Using congeneric information is generally not acceptable for this question but it may sometimes 

be prudent. For example, evidence that the entire genus/family/etc. is medicinally important or 

beneficial can be used as evidence for a “b” answer. In these cases, however, raise your 

uncertainty level. 

Notes 

 Review the same sources listed in Ent-1. 

 Try an internet search for “buy + taxon name.” 

 While Ent-1 covers only cultivation in the United States, in this question you should 

consider if the taxon is cultivated anywhere in the world. 

Full examples 

 Phyllanthus maderaspatensis is valued as a medicinal plant in several African countries 

(Schmelzer, 2008). “Generally grown from seed, but vegetative propagation by budding, 

grafting, cutting and root sprouting is possible...it is rarely cultivated...all plant parts are 

probably only collected from the wild. Mature plants...require little or no management 

once established” (Schmelzer, 2008). We are answering “c” with moderate uncertainty 

because we have evidence that this plant is being propagated, even if it is rarely done. 

Answer: “c” with moderate uncertainty. 

Ent-4 Entry as a contaminant 

Plants may also be introduced to new areas accidentally as contaminants during trade activities. 

Questions Ent-4a through Ent-4i evaluate the potential for a taxon to be introduced as a 

contaminant or hitchhiker of different pathways. 

Notes 

 Search published and online literature, as well available port interception records. 

 Positive evidence is required for a yes response to these questions. Answer no based on a 

lack of evidence. 

 Congeneric information is appropriate for questions Ent-4b through Ent-4i provided that 

the taxon being assessed is similar to the congeners and that the factors allowing the 

congener(s) to contaminate the pathway are expected to be similar for the taxon being 

assessed. Use higher uncertainty or answer unknown as appropriate. 
TP E-300, Ver. 2.3 February 11, 2019 114

Ent-4a Is the taxon present in Canada, Mexico, Central America, the Caribbean, or China? 

(y or n) 

Rationale. The likelihood of a plant being introduced into the United States as a contaminant is 

expected to be greater if the plant occurs in nearby regions and/or countries with which there is 

more trade or travel. If the taxon is present in these regions or countries, the scores from all of 

the contaminant-related questions are doubled. 

Criteria. Answer yes if a taxon is present and reasonably abundant in any of these countries or 

regions. Answer no if you find no evidence that the taxon occurs in those areas. 

Notes 

 Check VASCAN (http://data.canadensys.net/vascan/search/) 

 Although incomplete, the Flora of North America is a good source for determining 

presence in Canada: http://www.efloras.org/flora_page.aspx?flora_id=1

 Catalago de Malezas de Mexico (Villaseñor Rios and Espinosa Garcia, 1998) is good for 

weeds of Mexico but will not have cultivated plants. 

 The book series Flora Neotropica is useful for native and naturalized plants of the 

Caribbean and Central America (as well as tropical South America). 

 E-floras also feature an electronic flora of China: http://www.efloras.org/index.aspx. Also 

see Zhirong, 1990. 

Full examples 

 We found no evidence of Phyllanthus maderaspatensis being present in the United 

States, Canada, Central America, or the Caribbean. A single sample was collected from 

Hong Kong in the 19th century but otherwise this plant is not known to occur in China 

(eFlora, 2009). Answer: no with low uncertainty. 

References 

 Zhirong, W. (ed.). 1990. Farmland Weeds in China: A Collection of Coloured Illustrative 

Plates. Agricultural Publishing House. 506 pp. 

Ent-4b Is the plant taxon a contaminant of propagative plant material (except seed)? (y = 

0.4, n = 0, ? = 0)

Rationale. Propagative plant material is any material that is intended for reproducing and 

multiplying plants. Seeds are considered separately in the next question, so in this question 

evaluate nursery stock, roots, bulbs, corms, stems, or any other vegetative plant material. Weed 

propagules can be introduced to new areas as contaminants of propagative plant material. For 

example, roots of the weed Inula britannica grow intertwined with Hosta roots, which are 

imported into the United States. 

Criteria. Answer yes if you find evidence that the taxon may be in this pathway, otherwise 

answer no. 
TP E-300, Ver. 2.3 February 11, 2019 115

Full examples 

 Vegetative propagules of Crassula helmsii can contaminate aquatic plants in trade 

(OEPP/EPPO, 2007). Answer: yes with low uncertainty. 

Ent-4c Is the plant taxon a contaminant of seeds for planting? (y = 0.4, n = 0, ? = 0) 

Rationale. Seeds for planting is limited to any type of seed imported for planting, including (but 

not limited to) grain, grass, vegetable, and flower seeds. 

Criteria. Answer yes if you find evidence that the taxon may be in this pathway; otherwise, 

answer no. 

Notes 

 Search lists of noxious seeds, including USDA’s Germplasm Resource Information 

Network (GRIN) database (http://www.ars-grin.gov/) and the USDA manual of State 

Noxious Weed Seed Requirements 

(https://www.ams.usda.gov/sites/default/files/media/NWS%20List%202016.PDF). If the 

taxon appears on a noxious weed seed list, it is reasonable to assume the taxon is a 

contaminant of seed. When answering using this evidence alone, use high uncertainty. 

 Also review the information collected for ES-16. 

Full examples 

 Nassella neesiana is thought to have been introduced to the Waipawa area of New 

Zealand in contaminated pasture seed (Slay et al., 1999). Answer: yes with low 

uncertainty. 

Ent-4d Is the plant taxon a contaminant of ballast? (y = 0.3, n = 0, ? = 0)

Rationale. Ballast is a known pathway for many organisms, including aquatic plants, algae, and 

terrestrial plants that occur in coastal habitats (Drake and Lodge, 2007). Ballast includes material 

carried in the ballast tanks of ships or other vehicles to provide balance and stability. This may 

include water taken on at one port and released at new ports as cargo is offloaded, as well as dry 

ballast, such as rocks and sand. Dry ballast was very common prior to the twentieth century, but 

is probably less commonly used today. 

Criteria. Answer yes if you find evidence that the taxon may be in this pathway, otherwise 

answer no. 

Full examples 

 We found no evidence that Austroderia richardii propagules spread in ballast water, but 

noted this species occurs in riparian and coastal habitats (GBIF, 2013; Landcare 

Research, 2014; Linder et al., 2010; Parsons and Cuthbertson, 2001). Thus, its seeds may 

be taken up in ballast water. Answer: no with high uncertainty.
TP E-300, Ver. 2.3 February 11, 2019 116

References 

 Drake, J. M., and D. M. Lodge. 2007. Rate of species introductions in the Great Lakes via 

ships' ballast water and sediments. Canadian Journal of Fisheries and Aquatic Sciences 

64(3):530-538. 

Imp-4e Is the plant taxon a contaminant of aquarium plants or other aquarium products? 

(y = 0.2, n = 0, ? = 0)

Rationale. Contaminants of aquarium plants, live rock, or water used to transport them can be a 

pathway for certain plants and algae, such as Caulerpa spp. Contaminants may be introduced by 

people dumping aquarium contents into local environments. 

Criteria. Answer yes if you find evidence that the taxon may be in this pathway; otherwise, 

answer no. 

Full examples 

 Crassula helmsii is a contaminant of water plants (OEPP/EPPO, 2007) and can be spread 

to new areas via emptied aquaria (Dawson, 1994). Answer: yes with low uncertainty. 

Ent-4f Is the plant taxon a contaminant of imported landscape products? (y = 0.2, n = 0, ? = 

0)

Rationale. Imported landscape products include any materials used to improve the design of a 

landscape (e.g., pine straw, wood mulch, hydro mulch, gravel, top soil). These materials are an 

important weed contaminant pathway because landscape materials are often used in disturbed, 

open habitats that are favorable for plant establishment. 

Criteria. Answer yes if you find evidence that the taxon may be in the pathway; otherwise, 

answer no. 

Notes 

 Only consider materials that are incorporated into landscapes. Weeds that contaminate 

shipping pallets, tile, or other products that may be stored or set on the ground are 

considered in the next question, Ent-4g. 

Full examples 

 Unknown for Cestrum laevigatum. The related species C. parqui can be moved to new 

areas in roadside gravel when root pieces are relocated during cultivation or roadside 

grading (DAFF, 2013). Answer: unknown with maximum uncertainty.

Ent-4g Is the plant taxon a contaminant of containers, packing materials, equipment, 

conveyances, or other non-agricultural products? (y = 0.2, n = 0, ? = 0) 

Rationale. This category is very broad and is meant to include a variety of pathways. 

“Containers” refers to any kind of shipping container or materials used in shipping (e.g., 

refrigerated container units). “Equipment” includes engines, generators, boots and shoes, or other 
TP E-300, Ver. 2.3 February 11, 2019 117

tools. “Conveyances” refer to anything that serves as a means of transporting commodities 

including planes, railway cars, ships, trucks, and other vehicles. “Non-agricultural products” 

includes any non-plant material that is imported or traded, including tile, engines, etc. 

Criteria. Answer yes if you find evidence that the taxon may be in this pathway; otherwise, 

answer no. 

Full examples 

 A large population of Nassella neesiana in southern France may have been introduced 

through railway traffic (Verloove, 2005). Answer: yes with high uncertainty. 

Ent-4h Is the plant taxon a contaminant of highly processed commodities or fruits, 

vegetables, grains or other products for consumption? (y = 0.1, n = 0, ? = 0)

Rationale. Examples of products for consumption or processing that can be contaminated with 

propagules include grain for milling (e.g., wheat), seed for processing (e.g., spices), wool, and 

coconut coir. Relative to the previous pathways, this pathway poses less risk, because these 

products are either consumed or destroyed during processing. However, contaminants can still 

rarely escape during transport (e.g., grain spillage), or be removed and disposed of improperly 

(e.g., Nesom, 2004). 

Criteria. Answer yes if you find evidence that the taxon may be in this pathway; otherwise, 

answer no. 

Notes 

 Do not consider consumption commodities here if the intended use will place it directly 

into the environment where exotic taxa can readily escape and establish (e.g., hay, 

birdseed). In some of these cases, this information may be appropriate for Ent-4i. 

Full examples 

 Some French infestations of Nassella neesiana may have been introduced in imported 

cereals for consumption from Argentina (Verloove, 2005) Answer: yes with low 

uncertainty. 

Ent-4i Is the plant taxon a contaminant of some other pathway(s) not covered above? 

Choose the letter for the risk score that best represents the relative risk for the pathway(s): 

(a) 0; (b) 0.01; (c) 0.02; (d) 0.03; (e) 0.04. 

Rationale. This is a catchall question designed for any pathways not covered above and/or for 

evidence that did not readily fit the previous questions. 

Criteria. Clearly describe the pathway(s) in the notes column and provide supporting 

documentation. Choose the letter for the risk score that you think is appropriate for the 

pathway(s). If no other pathways are known, answer “a.” Consider the possible scores for other 

pathways described under Ent-4. The highest scores are reserved for contaminants in pathways 

that end in environments highly conducive for establishment (e.g., flower gardens, landscapes). 
TP E-300, Ver. 2.3 February 11, 2019 118

Scores are lower for pathways that are less likely to end in favorable environments, or in which 

propagules must overcome additional barriers to establishment. For example, aquarium plants 

must be dumped in an outdoor aquatic environment to establish. Answer unknown as 

appropriate. 

Notes 

 Recall that if the weed is present in any of the countries or regions indicated in Ent-4a 

this score value will be doubled. 

Full examples 

 Inflorescences of Austroderia richardii are used in dried floral arrangements 

(Anonymous, 2014b). Dried plumes of pampas grass (exact species is unknown) are 

available for sale on the internet (e.g., Anonymous, 2013b). Because A. richardii is also 

known as New Zealand pampas grass (Richardson et al., 2006), its flowers and associated 

seeds are likely to move in trade. Answer: “c” with low uncertainty. 

 Nassella neesiana is a contaminant of hay (Weller et al., 2012). We answered e for 0.4 

points because hay is used in agricultural environments where it could easily establish. 

Answer: “e” with negligible uncertainty. 

 We found no evidence that Phyllanthus maderaspatensis is a contaminant of other 

pathways. Answer: “a” with moderate uncertainty. 

Ent-5 Is the plant taxon likely to enter the United States through natural dispersal? (y = 0.6, 

n = 0, ? = 0) 

Rationale. As a pathway for entry, natural dispersal is most likely to occur for plants that occur 

in neighboring regions. For the lower 48 states, natural dispersal is likely for plants that grow 

along the Canadian and Mexican border, as well as for plants present in the upper Caribbean. 

Criteria. This question requires the risk analyst to use their best judgment. To answer yes, 

evidence must indicate that a taxon is present in a nearby region AND that it has a means of 

natural dispersal that enables long-distance colonization. Answer no if the taxon is either not 

present in a nearby country or if it has no relevant means of natural dispersal. Answer unknown 

if you find very little information about the taxon. 

Full examples 

 Austroderia richardii is wind-dispersed (Parsons and Cuthbertson, 2001) and the 

botanical garden where it is grown in British Colombia is relatively close to the U.S. 

border (GBIF, 2013). “The seed-bearing florets of the female plant [of the related species 

Cortaderia selloana] are particularly hairy and readily dispersed by wind, often being 

carried for up to 25 km“ (Parsons and Cuthbertson, 2001). Because the inflorescences of 

both species are similar (plumose, silky, and hairy; Landcare Research, 2014), we believe 

A. richardii is just as likely to enter the United States from Canada through wind 

dispersal. Answer: yes with high uncertainty. 

 Nassella neesiana does not seem likely to enter the United States through natural 

dispersal because it is not present in a bordering country. Answer: no with low 

uncertainty. 
TP E-300, Ver. 2.3 February 11, 2019 119

Appendix I. Conceptual process for the uncertainty analysis in the PPQ WRA 

The uncertainty analysis in the PPQ WRA uses a stochastic simulation to evaluate the sensitivity 

of the model-derived risk score to uncertainty in the assessment. The process described and 

shown below summarizes how the simulation works, using answers and uncertainties from the 

“Screening Tool” worksheet. The simulation is performed by @Risk within Microsoft Excel. 

After the full simulation is complete, the analyst will create the graph of the simulated risk scores 

around the observed score. The risk analysts and managers will interpret the results to make 

appropriate decisions. 

Step Process performed in the model 

1 For a given question, based on the uncertainty rating, randomly determine the 

probability that some other answer is correct. Probabilities are selected from 

triangular distributions, as defined by minimum, maximum, and most likely values 

(see Table 4, in Process 3). 

2 Using the probability from Step 1, determine in a binomial process if a given answer 

changes. For example, if the probability is 0.13, then the answer has a 13 percent 

chance of changing. 

3 If the answer changed, select a different answer. For yes/no questions this is a simple 

switch, while for multiple-choice questions another random determination may be 

necessary. 

4 Based on the answer, determine the risk score for that question. 

5 Repeat steps 1-4 for every question. 

6 Calculate the new risk scores for ES and Imp, determine the outcome of the WRA 

(low risk, evaluate further, or high risk), and run the secondary screening if necessary. 

7 Store and track the results. This represents one simulated WRA. 

8 Repeat Steps 1-7 4,999 more times to generate 5,000 simulated WRAs. 

 
TP E-300, Ver. 2.3 February 11, 2019 120

Figure I-1. Conceptual process of the stochastic simulation that is conducted as part of the PPQ 

WRA uncertainty process. 
TP E-300, Ver. 2.3 February 11, 2019 121

Appendix J. Work instructions for performing an uncertainty analysis (a 

stochastic simulation) using @Risk. 

1. After opening Microsoft Excel and the WRA workbook, open the @Risk program from 

Window’s Start button to load it into Excel. 

2. Go to the @Risk tab in the top menu tool bar. If this is the first time running @Risk or if 

the simulation settings have changed since the last WRA uncertainty analysis, do the 

following; otherwise go to Step 3. 

a. Click on the Simulation Settings icon. A new window will appear with several 

tabs. Change the indicated settings to the values described. Other settings are not 

relevant to a standard uncertainty analysis. 

b. General Tab 

i. Number of iterations: 5,000 

ii. Number of simulations: 1 

iii. Multiple CPU support: Enabled 

c. Sampling Tab 

i. Sampling type: Latin hypercube 

ii. Generator: Mersenne Twister 

iii. Initial seed: Fixed 101 

iv. Collect distribution samples: Inputs marked with collected 

d. View Tab 

i. Automatic results display: Show output graph 

ii. Update windows during simulation Automatic 

3. Close all other Excel workbooks containing @Risk models, including WRA workbooks 

for other taxa. This ensures that only one stochastic model is run at a time. 

4. Click on Start simulation. The simulation will only take a few moments to complete. 

5. In the @Risk menu tab, click on the “Simulation Detailed Statistics” icon under Results. 

Select and copy the means for the following columns. 

a. Proportion High Risk Output 

b. Proportion EF-High Risk Output 

c. Proportion EF-EF Risk Output 

d. Proportion EF-Low Risk Output 

e. Proportion Low Risk Output 

6. Go to the “Risk Score Plot” worksheet in the workbook. Paste the means in a blank space 

in the worksheet, and then enter the values as percentages (e.g., 23.2%) in the inset of the 

uncertainty graph (bottom graph). Ensure that the means are entered in the appropriate 

row corresponding to: High Risk, EFHigh, EFEF, EFLow, or Low. To avoid 

possible confusion, delete the values that you initially pasted. 

7. In the @Risk menu tab, click on the “Simulation Data” icon under Results. Select and 

copy columns 2 and 3 which correspond to “Establishment/Spread Output” and “Impact 

Potential Output.” Temporarily paste these two columns into two blank columns in the 

workbook. 

8. From the output just pasted into the spreadsheet, select the two cells in the first row 

containing simulated risk scores, and click on CTRL + SHIFT + Down Arrow. This 

selects all of the simulated risk scores. Copy the data. Click on the Home button on your 

keyboard, to quickly take you back to the top. Select cell E3, and paste the copied data. 
TP E-300, Ver. 2.3 February 11, 2019 122

9. Ensure that the uncertainty graph is properly displaying the data. To avoid possible 

confusion, delete the two columns of data that were temporarily pasted into the 

spreadsheet in Step 7. 

10. If the @Risk program loads from a shared network, select the Utilities button from the 

menu tab, and select “Unload @Risk Add-in” so that others may access it if there are a 

limited number of licenses in your organization. 

11. When saving or closing the Excel workbook, it is not necessary to save the @Risk results 

as they can be easily reproduced. 

 
TP E-300, Ver. 2.3 February 11, 2019 123

Acknowledgements 

The PPQ WRA Guidelines was completed through the dedicated efforts of the PERAL Weed 

Team and other PPQ employees who reviewed the document. We could not have completed 

these guidelines or developed the PPQ WRA process without the support of our supervisors and 

PPQ leaders. Much of the question-specific guidance used in this document is based on guidance 

developed by an international committee of participants at the 2nd International Weed Risk 

Assessment workshop in Perth, Australia, for the Australian WRA (Gordon et al., 2010; 

Pheloung et al., 1999)7. In many cases the guidance developed at this workshop was adapted, 

clarified, or augmented for the PPQ WRA process. We would like to thank Paul Pheloung and 

the editors of Plant Protection Quarterly for allowing us to use portions of those guidelines 

(Pheloung, 2009)8. 

Special thanks to members of the Canadian Food Inspection Agency’s weed risk analyst team, 

who not only reviewed this document as it was being created, but also inspired or created certain 

features of the guidelines. We also want to thank all past and future participants of our Weed 

Risk Assessment training workshops for their comments, which have helped to create and 

improve our guidelines. 

References within this document to commercial suppliers or products should not be construed as 

an endorsement of the company or product by the USDA. 

 

 

7

 Gordon, D. R., B. Mitterdorfer, P. C. Pheloung, S. Ansari, C. Buddenhagen, C. Chimera, C. C. Daehler, W. 

Dawson, J. S. Denslow, A. LaRosa, T. Nishida, D. A. Onderdonk, F. D. Panetta, P. Pyšek, R. P. Randall, D. M. 

Richardson, N. J. Tshidada, J. G. Virtue, and P. A. Williams. 2010. Guidance for addressing the Australian Weed 

Risk Assessment questions. Plant Protection Quarterly 25(2):56-74. 

Pheloung, P. C., P. A. Williams, and S. R. Halloy. 1999. A weed risk assessment model for use as a biosecurity tool 

evaluating plant introductions. Journal of Environmental Management 57:239-251. 

8

 Pheloung, P. 2009. Request to use question-specific guidance. Personal communication to A. Koop on December 

13, 2009, from Paul Pheloung, Department of Agriculture, Fisheries and Forestry, Australian Government. 
TP E-300, Ver. 2.3 February 11, 2019 124

Version Change Log 

This file is a controlled document under the PERAL Weed Team’s quality control system. 

Although the document is called “The PPQ WRA Guidelines”, it is filed as Technical Procedure 

E-300 (TP E-300). Version 2.2 of the PPQ WRA Guidelines was approved by Tony Koop. 

Below is a log of the changes made to the document since the original version. 

Date Description 

2/11/2019 Version 2.3 

 Removed the term ISO from one of the pages. 

9/29/2016 Version 2.2 

 Minor changes and updates to some of the citations related to plant 

hardiness zones. 

8/23/2016 Version 2.1 

 Formatted the Question specific guidance headings so that they show up 

in the Navigation pane (for ease of finding this guidance). 

 Re-inserted missing text from Section 1 (Instroduction to the PPQ Weed 

Risk Assessment) (We are not sure how this text got deleted from 

Version 2). 

6-3-2016 Version 2 

 Incorporated minor comments from weed risk analysts and WRA 

training workshop attendees. 

 Incorporated recommended minor changes from the PERAL Weed 

Team. 

 Improved consistency in the use of some terminology and formatting 

throughout the document. 

1-24-2015 Version 1 (Original)